Grid scroll: add diagonal line pattern

[pinobatch]

Upstream 240p Test Suite for Nintendo 64 has a new mode for the Grid scroll test. At 3:38 in the video, see the Grid background become diagonal stripes of width 4 pixels. Each cell in the grid is diagonal.png.

On NES, GB, and GBA, the Select Button is open for swapping this image. We'll probably need three states: white grid on black (current), diagonal, and black grid on white (invert grays).