XFB width/height handling fixes

A couple fixes to help games which mess with the width/height of the XFB (in Real XFB mode).  This doesn't implement horizontal scaling, so there will still be junk on the left side of the screen in games which use horizontal scaling, but this is a step in the right direction.  Vertical scaling should work correctly, and games which displayed gibberish using RealXFB should at least display something recognizable.

(I'm still working on the horizontal scaling fix, but I want to make sure this part doesn't cause any regressions.)