Tests: Add test for lossless webp file using a color indexing tranform

The image is https://quakewiki.org/wiki/File:Qpalette.png in lossless
webp format with a color indexing transform.

I've created Qpalette.webp by running

    examples/cwebp -z 0 ~/src/serenity/tmp.ppm -o Qpalette.webp

built at libwebp webmproject/libwebp@0825faa4c1 (without
png support, so I first ran

    Build/lagom/image ~/Downloads/Qpalette.png -o tmp.ppm

to convert it from png to a format my cwebp binary could read).

This file also happens to explicitly set max_symbol, so it serves
as a test for that code path as well.

Tests/LibGfx/TestImageDecoder.cpp

@@ -360,6 +360,26 @@ TEST_CASE(test_webp_extended_lossless)
     EXPECT_EQ(frame.image->get_pixel(359, 73), Gfx::Color(0, 0, 0, 128));
 }
 
+TEST_CASE(test_webp_simple_lossless_color_index_transform)
+{
+    // In addition to testing the index transform, this file also tests handling of explicity setting max_symbol.
+    auto file = MUST(Core::MappedFile::map(TEST_INPUT("Qpalette.webp"sv)));
+    EXPECT(Gfx::WebPImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = MUST(Gfx::WebPImageDecoderPlugin::create(file->bytes()));
+    EXPECT(plugin_decoder->initialize());
+
+    EXPECT_EQ(plugin_decoder->frame_count(), 1u);
+    EXPECT(!plugin_decoder->is_animated());
+    EXPECT(!plugin_decoder->loop_count());
+
+    EXPECT_EQ(plugin_decoder->size(), Gfx::IntSize(256, 256));
+
+    auto frame = MUST(plugin_decoder->frame(0));
+    EXPECT_EQ(frame.image->size(), Gfx::IntSize(256, 256));
+
+    EXPECT_EQ(frame.image->get_pixel(100, 100), Gfx::Color(0x73, 0x37, 0x23, 0xff));
+}
+
 TEST_CASE(test_webp_extended_lossless_animated)
 {
     auto file = MUST(Core::MappedFile::map(TEST_INPUT("extended-lossless-animated.webp"sv)));