graphics_1: rearrange, document.
graphics_1: clarify static vs. dynamic parameters.

This doesn't affect code generation.
graphics_1: special case 0/45/90-degree lines.

While not an *obvious* win, abstractly, this shaves off 2ms from the
worst case render performance of the rook demo, fixing an FPS
regression.
BUILD: answered an old question.

The answer is "yes, it's helpful."  Specifically, the performance of the
1bpp line drawing routines relies on the optimization level.
graphics_1: move line drawing into ROM.

It's actually faster this way, as it reduces contention for the SRAM
master port with the bitbanding unit.  This takes another 1300us off the
rook demo's worst case.
graphics_1: estimate likelihood of special cases.

This improves locality in the common case, shaving another ~350us
off the rook demo's frame time.
vga: make start_of_active_video early exit clear.