Changelog:

• Check for MIAF conformance and add as compatible brand.
• Signaling of premultiplied alpha.
• Parse AV1 obu_sequence_header for av1C box.
• Write pixi box in AVIFs.
• Save alpha as monochrome in AVIF if possible.
• Many build fixes.

Depends on D11702.

Signed-off-by: Martin Reboredo <yakoyoku@gmail.com>

Encoded two images into HEIF and AVIF respectively with magick and viewed the files with eog.