LibGfx/JPEG: More support for scans with a single component

Introduced in 2c98eff, support for non-interleaved scans was not working
for frames with a number of MCU per line or column that is odd. Indeed,
the decoder assumed that they have scans that include a fabricated MCU
like scans with multiple components.

This patch makes the decoder handle images with a number of MCU per line
or column that is odd. To do so, as in the current decoder state we do
not know if components are interleaved at allocation time, we skip over
falsely-created macroblocks when filling them. As stated in 2c98eff,
this is probably not a good solution and a whole refactor will be
welcome.

It also comes with a test that open a square image with a side of 600px,
meaning 75 MCUs.

Tests/LibGfx/TestImageDecoder.cpp

@@ -134,6 +134,17 @@ TEST_CASE(test_jpeg_sof2_spectral_selection)
     EXPECT_EQ(frame.image->size(), Gfx::IntSize(592, 800));
 }
 
+TEST_CASE(test_jpeg_sof0_several_scans_odd_number_mcu)
+{
+    auto file = MUST(Core::MappedFile::map(TEST_INPUT("several_scans_odd_number_mcu.jpg"sv)));
+    EXPECT(Gfx::JPEGImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = MUST(Gfx::JPEGImageDecoderPlugin::create(file->bytes()));
+    EXPECT(plugin_decoder->initialize());
+
+    auto frame = MUST(plugin_decoder->frame(0));
+    EXPECT_EQ(frame.image->size(), Gfx::IntSize(600, 600));
+}
+
 TEST_CASE(test_pbm)
 {
     auto file = MUST(Core::MappedFile::map(TEST_INPUT("buggie-raw.pbm"sv)));

Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp

@@ -446,9 +446,21 @@ static ErrorOr<void> build_macroblocks(JPEGLoadingContext& context, Vector<Macro
             for (u8 hfactor_i = 0; hfactor_i < scan_component.component.hsample_factor; hfactor_i++) {
                 // A.2.3 - Interleaved order
                 u32 macroblock_index = (vcursor + vfactor_i) * context.mblock_meta.hpadded_count + (hfactor_i + hcursor);
-                if (!context.current_scan.are_components_interleaved())
+                if (!context.current_scan.are_components_interleaved()) {
                     macroblock_index = vcursor * context.mblock_meta.hpadded_count + (hfactor_i + (hcursor * scan_component.component.vsample_factor) + (vfactor_i * scan_component.component.hsample_factor));
 
+                    // A.2.4 Completion of partial MCU
+                    // If the component is [and only if!] to be interleaved, the encoding process
+                    // shall also extend the number of samples by one or more additional blocks.
+
+                    // Horizontally
+                    if (macroblock_index >= context.mblock_meta.hcount && macroblock_index % context.mblock_meta.hpadded_count >= context.mblock_meta.hcount)
+                        continue;
+                    // Vertically
+                    if (macroblock_index >= context.mblock_meta.hpadded_count * context.mblock_meta.vcount)
+                        continue;
+                }
+
                 // G.1.2.2 - Progressive encoding of AC coefficients with Huffman coding
                 if (context.current_scan.end_of_bands_run_count > 0) {
                     --context.current_scan.end_of_bands_run_count;