ext_heif: Build libaom on Linux, remove dav1d/rav1e
We already use libaom instead of dav1d/rav1e for Windows and macOS on Binary Factory. Switching to libaom for Linux AppImage too can simplify our deps build setup a bit and remove the dependency on Meson and Rust.
I was told we used dav1d/rav1e because the performance and memory usage with libaom was pretty bad, but this seems to no longer apply and Lynx3d says libaom >= 3.2 performs pretty well. Any comments?