Many X11 scaling improvements...

1) Initialize the r2y array for xBR so it actually works.
2) Add a vertical (only) interpolation scaler for aspect ratio enforcement
3) Add a simple muliplier scaler, so that can be removed from x_event.c
4) Use a new graphics buffer free list, which allows tracking last
   drawn screen instead of last bitmap rectangle, removing various hacks
5) Share the Y'CbCr <-> R'dG'dB'd tables between xBR and scale.c