Animate the xor pattern and fix the resulting bug.
Animate the xor pattern and fix the resulting bug.

I started the pattern scrolling and it became really obvious that
rasterizing was glitching.  Turns out that my naive xor pattern
generator was taking more than its allotted time, causing lines to
be intermittently duplicated.  I've unrolled it just enough to
cut its execution time in half, and now things are smooth.

cbiffle/m4vgalib-demos,cbiffle/m4vgalib-demos,cbiffle/m4vgalib-demos