libheif: use libaom when dav1d and rav1e are not available
Windows Krita 5.0.0 beta 5 was built without AVIF support, unfortunately.
AVIF is supported via libheif which needs AV1 decoder and encoder to allow opening and saving AVIF.
Krita developers chose dav1d (as decoder) and rav1e (as encoder) in the past.
However, in case the preferred decoder and encoder are missing in the build environment, it is good to use at least google's libaom. Libaom was significantly improved meanwhile, it provides both encoder and decoder and it is very easy to build everywhere.