LibGfx: Implement SUBTRACT_GREEN_TRANSFORM for webp lossless decoder

Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp

@@ -636,6 +636,23 @@ public:
 
 Transform::~Transform() = default;
 
+// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#43_subtract_green_transform
+class SubtractGreenTransform : public Transform {
+public:
+    virtual ErrorOr<void> transform(Bitmap&) override;
+};
+
+ErrorOr<void> SubtractGreenTransform::transform(Bitmap& bitmap)
+{
+    for (ARGB32& pixel : bitmap) {
+        Color color = Color::from_argb(pixel);
+        u8 red = (color.red() + color.green()) & 0xff;
+        u8 blue = (color.blue() + color.green()) & 0xff;
+        pixel = Color(red, color.green(), blue, color.alpha()).value();
+    }
+    return {};
+}
+
 }
 
 // https://developers.google.com/speed/webp/docs/riff_container#simple_file_format_lossless
@@ -702,7 +719,8 @@ static ErrorOr<void> decode_webp_chunk_VP8L(WebPLoadingContext& context, Chunk c
         case COLOR_TRANSFORM:
             return context.error("WebPImageDecoderPlugin: VP8L COLOR_TRANSFORM handling not yet implemented");
         case SUBTRACT_GREEN_TRANSFORM:
-            return context.error("WebPImageDecoderPlugin: VP8L SUBTRACT_GREEN_TRANSFORM handling not yet implemented");
+            TRY(transforms.try_append(TRY(try_make<SubtractGreenTransform>())));
+            break;
         case COLOR_INDEXING_TRANSFORM:
             return context.error("WebPImageDecoderPlugin: VP8L COLOR_INDEXING_TRANSFORM handling not yet implemented");
         }