Use MythAVFrame convenience class
OSX: Allow PiP with VDA decoder
MythUSWCCopy: Change const-ness of argument
Add MythAVCopy class

Currently, all frames conversions are done using myth_sws_img_convert, which in practice serialise all conversions made no matter how many threads we have running. This is extremely inefficient. Also, under typical use, myth_sws_img_convert will constantly delete and recreate the SwsContext.

This class will also centralise all future conversion optimisations
Remove unused header
Remove unused header
Remove any use of myth_sws_img_convert and use new MythAVCopy API

Remove myth_sws_img_convert API.

Most visible benefit is the reduction of stutter occurring in main video when one PiP is paused