Used by the routine at RUN_PICTURE.

A Bresenham stepper in two mirrored halves, picked by bit 0 of C: one walks x as the major axis, the other y. Bit 1 of C is the y direction and bit 2 the x direction. B counts down to the next minor-axis step and is reloaded from the stack each time it runs out; L counts the pixels.

It already steps the coordinates one at a time, so it knows exactly where the next pixel is -- and then calls PLOT_PIXEL, which throws that away and recomputes the address from the coordinates. Carrying the address and mask along the line instead is where the large win is.