reply on.  Make it harder to merge this code from upstream in the
future instead.

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

Uses "pointyscale" for x3 and x5
Uses xBR from FFmpeg for x2 and x4
HQx is also included, but unused as it's too slow at x4.