VideoCommon: remove XFReg copy optimization

This code is just ugly and I doubt there is a way that copying twice is faster.
VideoCommon: rewrite frame skipping code
Merge pull request #958 from degasus/xf_cleanup

VideoCommon: remove XFReg copy optimization
Merge pull request #957 from degasus/frame_skipping

VideoCommon: rewrite frame skipping code
Merge remote-tracking branch 'upstream/master'