LibGfx/JPEG: Allow decoding more subsampling factors

We now allow all subsampling factors where the subsampling factors
of follow-on components evenly decode the ones of the first component.

In practice, this allows YCCK 2111, CMYK 2112, and CMYK 2111.

Tests/LibGfx/TestImageDecoder.cpp

@@ -314,7 +314,7 @@ TEST_CASE(test_jpeg_ycck)
 {
     Array test_inputs = {
         TEST_INPUT("jpg/ycck-1111.jpg"sv),
-        // TEST_INPUT("jpg/ycck-2111.jpg"sv), // FIXME: Enable once this decodes correctly
+        TEST_INPUT("jpg/ycck-2111.jpg"sv),
         TEST_INPUT("jpg/ycck-2112.jpg"sv),
     };
 

Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp

@@ -1287,12 +1287,9 @@ static ErrorOr<void> read_start_of_frame(JPEGStream& stream, JPEGLoadingContext&
                 return Error::from_string_literal("Unsupported luma subsampling factors");
             }
         } else {
-            // YCCK with just CC subsampled and K matching Y is fine.
             auto const& y_component = context.components[0];
-            bool channel_matches_y_factor = component.sampling_factors == y_component.sampling_factors;
-            bool k_channel_matches_y = context.color_transform == ColorTransform::YCCK && i == 3 && channel_matches_y_factor;
-
-            if (((component.sampling_factors != SamplingFactors { 1, 1 }) && !k_channel_matches_y) || (i == 3 && !channel_matches_y_factor)) {
+            if (y_component.sampling_factors.horizontal % component.sampling_factors.horizontal != 0
+                || y_component.sampling_factors.vertical % component.sampling_factors.vertical != 0) {
                 dbgln_if(JPEG_DEBUG, "Unsupported chroma subsampling factors: horizontal: {}, vertical: {}",
                     component.sampling_factors.horizontal,
                     component.sampling_factors.vertical);
@@ -1589,8 +1586,7 @@ static void inverse_dct(JPEGLoadingContext const& context, Vector<Macroblock>& m
 static void undo_subsampling(JPEGLoadingContext const& context, Vector<Macroblock>& macroblocks)
 {
     // The first component has sampling factors of context.sampling_factors, while the others
-    // are either 1x1 or for the 4th component also context.sampling_factors. See
-    // read_start_of_frame() which currently enforces these restrictions.
+    // are divide the first component's sampling factors. This is enforced by read_start_of_frame().
     // This function undoes the subsampling by duplicating the values of the smaller components.
     // See https://www.w3.org/Graphics/JPEG/itu-t81.pdf, A.2 Order of source image data encoding.
     //