AVIF Support

Support for reading and writing AVIF (8 bit) images through libaom has been added.
Other encoders/decoders can be added with the plugin architecture.
Decoders don't need any code change to also read AVIF images.
Encoders can reuse the existing HEIF code and simply use heif_compression_AV1 to output AVIF.

Other changes:

• HDR image rotation
• HDR to 8 bit conversion
• accept images with only the 'mif1' brand
• bug fixes