Tests: Check that color indexing reduces file size

Tests/LibGfx/TestImageWriter.cpp

@@ -203,7 +203,16 @@ TEST_CASE(test_webp_color_indexing_transform)
             for (int x = 0; x < bitmap->width(); ++x)
                 bitmap->set_pixel(x, y, colors[(x * bitmap->width() + y) % number_of_colors]);
 
-        TRY_OR_FAIL((test_roundtrip<Gfx::WebPWriter, Gfx::WebPImageDecoderPlugin>(bitmap)));
+        auto encoded_data = TRY_OR_FAIL(encode_bitmap<Gfx::WebPWriter>(bitmap));
+        auto decoded_bitmap = TRY_OR_FAIL(expect_single_frame_of_size(*TRY_OR_FAIL(Gfx::WebPImageDecoderPlugin::create(encoded_data)), bitmap->size()));
+        expect_bitmaps_equal(*decoded_bitmap, *bitmap);
+
+        Gfx::WebPEncoderOptions options;
+        options.vp8l_options.allowed_transforms = 0;
+        auto encoded_data_without_color_indexing = TRY_OR_FAIL(encode_bitmap<Gfx::WebPWriter>(bitmap, options));
+        EXPECT(encoded_data.size() < encoded_data_without_color_indexing.size());
+        auto decoded_bitmap_without_color_indexing = TRY_OR_FAIL(expect_single_frame_of_size(*TRY_OR_FAIL(Gfx::WebPImageDecoderPlugin::create(encoded_data)), bitmap->size()));
+        expect_bitmaps_equal(*decoded_bitmap_without_color_indexing, *decoded_bitmap);
     }
 }