LibGfx/WebPLoader: Use transparent black as animation background color

This matches libwebp (see ZeroFillCanvas() call in
libwebp/src/demux/anim_decode.c:355 and ZeroFillFrameRect() call
in line 435, but in WebPAnimDecoderGetNext()) and makes files
written e.g. by asesprite look correct -- even though the old
behavior is also spec-compliant and arguably makes more sense.
Now nothing looks at the background color stored in the file.

See PR for an example image where it makes a visible difference.

Tests/LibGfx/TestImageDecoder.cpp

@@ -923,7 +923,7 @@ TEST_CASE(test_webp_extended_lossless_animated)
         EXPECT_EQ(frame.image->get_pixel(500, 700), Gfx::Color::Yellow);
 
         // This one isn't the same in all frames.
-        EXPECT_EQ(frame.image->get_pixel(500, 0), (frame_index == 2 || frame_index == 6) ? Gfx::Color::Black : Gfx::Color(255, 255, 255, 0));
+        EXPECT_EQ(frame.image->get_pixel(500, 0), (frame_index == 2 || frame_index == 6) ? Gfx::Color::Black : Gfx::Color(0, 0, 0, 0));
     }
 }