Add .avif (AV1 Image File Format) support

The AV1 Image File Format (AVIF) is a new image format based on the AV1 video codec. It would be great if support for this format could be added.

The AVIF spec can be found here: https://aomediacodec.github.io/av1-avif/

Netflix published a bunch of test images: http://download.opencontent.netflix.com/?prefix=AV1/Chimera/AVIF/

• Import/decode
• Export/encode