Tests: Add test for webp lossless decoding

Tests/LibGfx/TestImageDecoder.cpp

@@ -330,6 +330,22 @@ TEST_CASE(test_webp_extended_lossless)
     EXPECT(!plugin_decoder->loop_count());
 
     EXPECT_EQ(plugin_decoder->size(), Gfx::IntSize(417, 223));
+
+    auto frame = MUST(plugin_decoder->frame(0));
+    EXPECT_EQ(frame.image->size(), Gfx::IntSize(417, 223));
+
+    // Check some basic pixels.
+    EXPECT_EQ(frame.image->get_pixel(0, 0), Gfx::Color(0, 0, 0, 0));
+    EXPECT_EQ(frame.image->get_pixel(43, 75), Gfx::Color(255, 0, 0, 255));
+    EXPECT_EQ(frame.image->get_pixel(141, 75), Gfx::Color(0, 255, 0, 255));
+    EXPECT_EQ(frame.image->get_pixel(235, 75), Gfx::Color(0, 0, 255, 255));
+    EXPECT_EQ(frame.image->get_pixel(341, 75), Gfx::Color(0, 0, 0, 128));
+
+    // Check pixels using the color cache.
+    EXPECT_EQ(frame.image->get_pixel(94, 73), Gfx::Color(255, 0, 0, 255));
+    EXPECT_EQ(frame.image->get_pixel(176, 115), Gfx::Color(0, 255, 0, 255));
+    EXPECT_EQ(frame.image->get_pixel(290, 89), Gfx::Color(0, 0, 255, 255));
+    EXPECT_EQ(frame.image->get_pixel(359, 73), Gfx::Color(0, 0, 0, 128));
 }
 
 TEST_CASE(test_webp_extended_lossless_animated)