Test suite to run images/ with --enable-gpu-rasterization.

This suite tests YUV decoding and rasterization for images that have been fully loaded at decode time and formats that support it, e.g.

• lossy WebP images without alpha that and
• JPEG YUV images with a 4:2:0 subsampling.

Because incremental YUV decoding hasn't been implemented (crbug.com/943519), the WebP tests use JavaScript to ensure all image data has been received before we attempt decoding.