LibGfx+Tests: Improve calculation of restart interval

JPEGs can store a `restart_interval`, which controls how many
minimum coded units (MCUs) apart the stream state resets.
This can be used for error correction, decoding parts of a jpeg
in parallel, etc.

We tried to use

    u32 i = vcursor * context.mblock_meta.hpadded_count + hcursor;
    i % (context.dc_restart_interval *
         context.sampling_factors.vertical *
         context.sampling_factors.horizontal) == 0

to check if we hit a multiple of an MCU.

`hcursor` is the horizontal offset into 8x8 blocks, vcursor the
vertical offset, and hpadded_count stores how many 8x8 blocks
we have per row, padded to a multiple of the sampling factor.

This isn't quite right if hcursor isn't divisible by both
the vertical and horizontal sampling factor. Tweak things so
that they work.

Also rename `i` to `number_of_mcus_decoded_so_far` since that
what it is, at least now.

For the test case, I converted an existing image to a ppm:

    Build/lagom/bin/image -o out.ppm \
        Tests/LibGfx/test-inputs/jpg/12-bit.jpg

Then I resized it to 102x77px in Photoshop and saved it again.
Then I turned it into a jpeg like so:

    path/to/cjpeg \
        -outfile Tests/LibGfx/test-inputs/jpg/odd-restart.jpg \
        -sample 2x2,1x1,1x1 -quality 5 -restart 3B out.ppm

The trick here is to:

a) Pick a size that's not divisible by the data size width (8),
   and that when rounded to a block size (13) still isn't divisible
   by the subsample factor -- done by picking a width of 102.
b) Pick a huffman table that doesn't happen to contain the bit
   pattern for a restart marker, so that reading a restart marker
   from the bitstream as data causes a failure (-quality 5 happens
   to do this)
c) Pick a restart interval where we fail to skip it if our calculation
   is off (-restart 3B)

Together with #22987, fixes #22780.

Tests/LibGfx/TestImageDecoder.cpp

@@ -324,6 +324,15 @@ TEST_CASE(test_jpeg_sof0_several_scans)
     TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 592, 800 }));
 }
 
+TEST_CASE(test_odd_mcu_restart_interval)
+{
+    auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("jpg/odd-restart.jpg"sv)));
+    EXPECT(Gfx::JPEGImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = TRY_OR_FAIL(Gfx::JPEGImageDecoderPlugin::create(file->bytes()));
+
+    TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 102, 77 }));
+}
+
 TEST_CASE(test_jpeg_rgb_components)
 {
     auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("jpg/rgb_components.jpg"sv)));

Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp

@@ -798,12 +798,14 @@ static ErrorOr<void> decode_huffman_stream(JPEGLoadingContext& context, Vector<M
 {
     for (u32 vcursor = 0; vcursor < context.mblock_meta.vcount; vcursor += context.sampling_factors.vertical) {
         for (u32 hcursor = 0; hcursor < context.mblock_meta.hcount; hcursor += context.sampling_factors.horizontal) {
-            u32 i = vcursor * context.mblock_meta.hpadded_count + hcursor;
+            // FIXME: This is likely wrong for non-interleaved scans.
+            VERIFY(context.mblock_meta.hpadded_count % context.sampling_factors.horizontal == 0);
+            u32 number_of_mcus_decoded_so_far = ((vcursor / context.sampling_factors.vertical) * context.mblock_meta.hpadded_count + hcursor) / context.sampling_factors.horizontal;
 
             auto& huffman_stream = context.current_scan->huffman_stream;
 
             if (context.dc_restart_interval > 0) {
-                if (i != 0 && i % (context.dc_restart_interval * context.sampling_factors.vertical * context.sampling_factors.horizontal) == 0) {
+                if (number_of_mcus_decoded_so_far != 0 && number_of_mcus_decoded_so_far % context.dc_restart_interval == 0) {
                     reset_decoder(context);
 
                     // Restart markers are stored in byte boundaries. Advance the huffman stream cursor to
@@ -823,7 +825,7 @@ static ErrorOr<void> decode_huffman_stream(JPEGLoadingContext& context, Vector<M
 
             if (result.is_error()) {
                 if constexpr (JPEG_DEBUG) {
-                    dbgln("Failed to build Macroblock {}: {}", i, result.error());
+                    dbgln("Failed to build Macroblock {}: {}", number_of_mcus_decoded_so_far, result.error());
                     dbgln("Huffman stream byte offset {}", context.stream.byte_offset());
                 }
                 return result.release_error();