Add libdovi dependency and mapping helper

Added pl_frame_map_avdovi_metadata helper for adding DOVI metadata from AVFrame side data. To simplify the mapping for users as otherwise they need to copy the implementation.

Added libdovi dependency and dovi_rpu_helper single header/source file, to extract dynamic metadata. Currently only infers scene_max/scene_avg for tone mapping.

And use pl_frame_map_dovi_from_rpu_buffer in pl_map_avframe_internal, as we can access the RPU buffer there.

Will probably make the changes for mpv in https://github.com/mpv-player/mpv/pull/11307.

I'm just not sure if we should always include dovi_rpu_helper.h regardless of if libdovi is found.