WebP/Lossless: Set alpha to 0xff if is_alpha_used is false in header

simple-vp8l-alpha-used-false.webp is a copy of simple-vp8l.webp,
with the byte at offset 0x18 changed from 0x10 to 0x00 -- that
is, the bit in the VP8L header that stores `is_alpha_used` is cleared.

We would already allocated a BGRx8888 instead of a BGRA8888 bitmap,
but keep actual alpha data in the `x` channel.

That lead to at least `image` still writing a PNG with an alpha channel.
So explicitly set the alpha channel to 0xff when is_alpha_used is false,
to make sure all consumers of decoded lossless webp data have behavior
consistent with other webp readers.

In practice, webp encoders usually don't write files that have
`is_alpha_used` set to false and then write actual alpha data to their
output. So this is rarely observable. However, for example for
lossy+ALPH webp files, the lossless webp used to store the ALPH channel
has `is_alpha_used` set to false and all channels but green are 0
(since the lossless green channel stores the alpha channel of a
lossy+ALPH webp). So if we dump such a bitmap to a standalone webp
file (e.g. with the temporary debugging code in fc3249a1ca),
then without this commit here, `image` would convert that webp to
a fully transparent webp, while other webp software would correctly
display the green image with opaque alpha.

Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossless.cpp

@@ -1000,6 +1000,9 @@ ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_contents(VP8LHeader const&
     for (auto const& transform : transforms.in_reverse())
         bitmap = TRY(transform->transform(bitmap));
 
+    if (!vp8l_header.is_alpha_used)
+        bitmap->strip_alpha_channel();
+
     return bitmap;
 }