GPU-accelerated text/subtitle rendering
Currently FFmpeg/MPV doesn't have GPU-based subtitle rendering and burning filters, so you have to use CPU filters via hwdownload, which seriously affects the efficiency.
This FR requests the ability to render text using the GPU, and exposes this functionality in FFmpeg's vf_libplacebo, replacing the ass and subtitles filters for fully hardware-accelerated transcoding + subtitle burning use cases.
This feature may also be reused for mpv to implement a new subtitle rendering mechanism.