LibGfx: Enable webp lossless Transform to return new bitmap

...in addition to modifying in-place. This is needed for bitpacking
support for the color indexing transform (and it could also be used
to make the color indexing transform return an indexed bitmap, which
is something we could do if that's the last transform that's applied).

No behavior change.

Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp

@@ -750,7 +750,9 @@ static ARGB32 add_argb32(ARGB32 a, ARGB32 b)
 class Transform {
 public:
     virtual ~Transform();
-    virtual ErrorOr<void> transform(Bitmap&) = 0;
+
+    // Could modify the input bitmap and return it, or could return a new bitmap.
+    virtual ErrorOr<NonnullRefPtr<Bitmap>> transform(NonnullRefPtr<Bitmap>) = 0;
 };
 
 Transform::~Transform() = default;
@@ -759,7 +761,7 @@ Transform::~Transform() = default;
 class PredictorTransform : public Transform {
 public:
     static ErrorOr<NonnullOwnPtr<PredictorTransform>> read(WebPLoadingContext&, LittleEndianInputBitStream&, IntSize const& image_size);
-    virtual ErrorOr<void> transform(Bitmap&) override;
+    virtual ErrorOr<NonnullRefPtr<Bitmap>> transform(NonnullRefPtr<Bitmap>) override;
 
 private:
     PredictorTransform(int size_bits, NonnullRefPtr<Bitmap> predictor_bitmap)
@@ -857,8 +859,10 @@ ErrorOr<NonnullOwnPtr<PredictorTransform>> PredictorTransform::read(WebPLoadingC
     return adopt_nonnull_own_or_enomem(new (nothrow) PredictorTransform(size_bits, move(predictor_bitmap)));
 }
 
-ErrorOr<void> PredictorTransform::transform(Bitmap& bitmap)
+ErrorOr<NonnullRefPtr<Bitmap>> PredictorTransform::transform(NonnullRefPtr<Bitmap> bitmap_ref)
 {
+    Bitmap& bitmap = *bitmap_ref;
+
     // "There are special handling rules for some border pixels.
     //  If there is a prediction transform, regardless of the mode [0..13] for these pixels,
     //  the predicted value for the left-topmost pixel of the image is 0xff000000,
@@ -908,7 +912,7 @@ ErrorOr<void> PredictorTransform::transform(Bitmap& bitmap)
 
         bitmap_previous_scanline = bitmap_scanline;
     }
-    return {};
+    return bitmap_ref;
 }
 
 ErrorOr<ARGB32> PredictorTransform::predict(u8 predictor, ARGB32 TL, ARGB32 T, ARGB32 TR, ARGB32 L)
@@ -980,7 +984,7 @@ ErrorOr<ARGB32> PredictorTransform::predict(u8 predictor, ARGB32 TL, ARGB32 T, A
 class ColorTransform : public Transform {
 public:
     static ErrorOr<NonnullOwnPtr<ColorTransform>> read(WebPLoadingContext&, LittleEndianInputBitStream&, IntSize const& image_size);
-    virtual ErrorOr<void> transform(Bitmap&) override;
+    virtual ErrorOr<NonnullRefPtr<Bitmap>> transform(NonnullRefPtr<Bitmap>) override;
 
 private:
     ColorTransform(int size_bits, NonnullRefPtr<Bitmap> color_bitmap)
@@ -1015,8 +1019,10 @@ ErrorOr<NonnullOwnPtr<ColorTransform>> ColorTransform::read(WebPLoadingContext&
     return adopt_nonnull_own_or_enomem(new (nothrow) ColorTransform(size_bits, move(color_bitmap)));
 }
 
-ErrorOr<void> ColorTransform::transform(Bitmap& bitmap)
+ErrorOr<NonnullRefPtr<Bitmap>> ColorTransform::transform(NonnullRefPtr<Bitmap> bitmap_ref)
 {
+    Bitmap& bitmap = *bitmap_ref;
+
     for (int y = 0; y < bitmap.height(); ++y) {
         ARGB32* bitmap_scanline = bitmap.scanline(y);
 
@@ -1028,7 +1034,7 @@ ErrorOr<void> ColorTransform::transform(Bitmap& bitmap)
             bitmap_scanline[x] = inverse_transform(bitmap_scanline[x], color_scanline[color_x]);
         }
     }
-    return {};
+    return bitmap_ref;
 }
 
 ARGB32 ColorTransform::inverse_transform(ARGB32 pixel, ARGB32 transform)
@@ -1060,25 +1066,25 @@ ARGB32 ColorTransform::inverse_transform(ARGB32 pixel, ARGB32 transform)
 // https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#43_subtract_green_transform
 class SubtractGreenTransform : public Transform {
 public:
-    virtual ErrorOr<void> transform(Bitmap&) override;
+    virtual ErrorOr<NonnullRefPtr<Bitmap>> transform(NonnullRefPtr<Bitmap>) override;
 };
 
-ErrorOr<void> SubtractGreenTransform::transform(Bitmap& bitmap)
+ErrorOr<NonnullRefPtr<Bitmap>> SubtractGreenTransform::transform(NonnullRefPtr<Bitmap> bitmap)
 {
-    for (ARGB32& pixel : bitmap) {
+    for (ARGB32& pixel : *bitmap) {
         Color color = Color::from_argb(pixel);
         u8 red = (color.red() + color.green()) & 0xff;
         u8 blue = (color.blue() + color.green()) & 0xff;
         pixel = Color(red, color.green(), blue, color.alpha()).value();
     }
-    return {};
+    return bitmap;
 }
 
 // https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#44_color_indexing_transform
 class ColorIndexingTransform : public Transform {
 public:
     static ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> read(WebPLoadingContext&, LittleEndianInputBitStream&);
-    virtual ErrorOr<void> transform(Bitmap&) override;
+    virtual ErrorOr<NonnullRefPtr<Bitmap>> transform(NonnullRefPtr<Bitmap>) override;
 
 private:
     explicit ColorIndexingTransform(NonnullRefPtr<Bitmap> palette_bitmap)
@@ -1113,18 +1119,18 @@ ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> ColorIndexingTransform::read(WebP
     return adopt_nonnull_own_or_enomem(new (nothrow) ColorIndexingTransform(move(palette_bitmap)));
 }
 
-ErrorOr<void> ColorIndexingTransform::transform(Bitmap& bitmap)
+ErrorOr<NonnullRefPtr<Bitmap>> ColorIndexingTransform::transform(NonnullRefPtr<Bitmap> bitmap)
 {
     // FIXME: If this is the last transform, consider returning an Indexed8 bitmap here?
 
-    for (ARGB32& pixel : bitmap) {
+    for (ARGB32& pixel : *bitmap) {
         // "The inverse transform for the image is simply replacing the pixel values (which are indices to the color table)
         //  with the actual color table values. The indexing is done based on the green component of the ARGB color. [...]
         //  If the index is equal or larger than color_table_size, the argb color value should be set to 0x00000000 (transparent black)."
         u8 index = Color::from_argb(pixel).green();
         pixel = index < m_palette_bitmap->width() ? m_palette_bitmap->scanline(0)[index] : 0;
     }
-    return {};
+    return bitmap;
 }
 
 }
@@ -1209,7 +1215,7 @@ static ErrorOr<void> decode_webp_chunk_VP8L(WebPLoadingContext& context, Chunk c
     // Transforms have to be applied in the reverse order they appear in in the file.
     // (As far as I can tell, this isn't mentioned in the spec.)
     for (auto const& transform : transforms.in_reverse())
-        TRY(transform->transform(*context.bitmap));
+        context.bitmap = TRY(transform->transform(context.bitmap.release_nonnull()));
 
     return {};
 }