Clarify TODO
Microbenchmark and then pick either the set() or the explicit array copy. Chrome seems to be faster in the latter, but Firefox is terrible at that. Firefox is still slow, but this is hopefully a start.
Disable screen output when r6 = 0.

Closes #41
Blank left border when the startX() changes.

Fixes boffin's screen on ctrl-break. I haven't put in an equivalent
right blank as it's trickier to get right and without a test case I'm
not confident I'd get it correct. Plus there's plenty of blanking
overhang on the right.

Partially addresses #42 (the Exile part is still valid).
Cache parts of the video memory address.

Greatly simplifies the code paths in the hot part of the video renderer,
but doesn't seem to have the speed impact I'd have hoped it would have.
Checking in despite a few % slowdown (in some runs) as I morally feel it
is a better approach and hopefully opens the door to higher level
optimizations higher up the call stack.
Change instruction dispatch for FF.

Uses a jump table for FF, much faster. Still not as fast as Chrome,
but it's getting there.  Helps issue #12