This can be used to make vo_libmpv render video to a memory buffer. It
only adds a new backend API that takes memory surfaces. All the render
API (such as frame rendering control and so on) is reused.
I'm not quite convinced of the usefulness of this, and until now I
always resisted providing something like this. It only seems to
facilitate inefficient implementation. But whatever.
Unfortunately, this duplicates the software rendering glue code yet
again (like it exists in vo_x11, vo_wlshm, vo_drm, and probably more).
But in theory, these could reuse this backend in the future, just like
vo_gpu could reuse the render_gl API.
Fixes: #7852
wm4