math: factor geometry operations out of demo/rook.
math: interpolation, vector math.
demo/xor_pattern: take direct width value.

This was the last Rasterizer that still wanted knowledge of our timing
configuration, which made it a little harder to reuse.
demo/rotozoom: first draft.
demo/rook: remove edge glitches in text scrolling.
demo/wipe: more interesting mixed modes.