mpv backend relies on qthelper.hpp header, which was removed in mpv 0.33.0. While it's possible to provide this header in source tree of Subtitle Composer (that's what mpv recommends downstreams to do), I think it's not necessary:
1. GStreamer backend is still functional and on par with mpv backend.
2. Upstream Subtitle Composer don't even support these backends anymore - they have removed them in favor of unified backend based on QtAV.
Depends on D10020.