avcodec/videotoolbox: allow not setting the kCVPixelBufferPixelFormatTypeKey

If AVVideotoolboxContext.cv_pix_fmt_type is set to 0, don't set the
kCVPixelBufferPixelFormatTypeKey value on the VT decoder.

This makes VT output its native format, which can be much faster on
some hardware iterations (if the native format does not match with
the requested format, it will be converted, which is slow).

The default is still forcing nv12.