LibGfx: Properly skip IDAT chunks without fcTL in APNG files

In the previous fix, we were still drawing IDAT data to the reference
frame even when no fcTL was present. This would cause rendering issues
when subsequent frames use APNG_BLEND_OP_OVER blending mode, as they
would composite over the incorrect reference frame. This commit adds a
simple check to properly skip any frame without an fcTL chunk.

Libraries/LibGfx/ImageFormats/PNGLoader.cpp

@@ -301,6 +301,9 @@ ErrorOr<size_t> PNGLoadingContext::read_frames(png_structp png_ptr, png_infop in
 
             auto decoded_frame_bitmap = TRY(decode_frame({ width, height }));
 
+            if (!has_fcTL)
+                continue;
+
             RefPtr<Bitmap> prev_output_buffer;
             if (dispose_op == PNG_DISPOSE_OP_PREVIOUS) // Only actually clone if it's necessary
                 prev_output_buffer = TRY(output_buffer->clone());
@@ -318,10 +321,8 @@ ErrorOr<size_t> PNGLoadingContext::read_frames(png_structp png_ptr, png_infop in
                 VERIFY_NOT_REACHED();
             }
 
-            if (has_fcTL) {
-                animation_frame_count++;
-                frame_descriptors.append({ TRY(output_buffer->clone()), duration_ms() });
-            }
+            animation_frame_count++;
+            frame_descriptors.append({ TRY(output_buffer->clone()), duration_ms() });
 
             switch (dispose_op) {
             case PNG_DISPOSE_OP_NONE:

Tests/LibGfx/TestImageDecoder.cpp

@@ -421,6 +421,22 @@ TEST_CASE(test_apng)
     EXPECT_EQ(frame.image->size(), Gfx::IntSize(128, 64));
 }
 
+TEST_CASE(test_apng_idat_not_affecting_next_frame)
+{
+    auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("png/apng-blend.png"sv)));
+    EXPECT(Gfx::PNGImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = TRY_OR_FAIL(Gfx::PNGImageDecoderPlugin::create(file->bytes()));
+
+    EXPECT_EQ(plugin_decoder->frame_count(), 1u);
+    EXPECT_EQ(plugin_decoder->loop_count(), 0u);
+
+    auto frame = TRY_OR_FAIL(plugin_decoder->frame(0));
+
+    EXPECT_EQ(frame.duration, 1000);
+    EXPECT_EQ(frame.image->get_pixel(0, 0), Gfx::Color::NamedColor::Transparent);
+    EXPECT_EQ(frame.image->size(), Gfx::IntSize(100, 100));
+}
+
 TEST_CASE(test_exif)
 {
     auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("png/exif.png"sv)));