Add libde265 and libx265 plugins for libheif in ffmpeg extension

Ship with libde265 and libx265 plugins in ffmpeg-full

Closes #1704 (closed)

Someone should test if the libheif plugin configuration actually works in flatpak. I have no way to test that unless it is merged. I only checked that it builds and the contents of the build artifacts are fine + ldd - foo on them.