The new API has literally no advantages (other than that we can drop
mp_vt_download_image and other things later), but it's sort-of uniform
with the other hwaccels.
"--videotoolbox-format=no" is not supported with the new API, because it
doesn't "fit in". Probably could be added later again.
The iOS code change is untested (no way to test).
Implements --hwdec=videotoolbox on iOS. Similar to hwdec_osx.c, but
using CVPixelBuffer APIs available on iOS instead of the equivalent
IOSurface APIs in macOS.