CMD_DRAW_LINE (23) implements Bresenham's algorithm with support for line thickness and dashed style. The command takes 5 parameters: start X/Y, end X/Y, color, thickness, and dash style.

Fixed two bugs:
- thick_step: stray pixels when drawing thick lines — used $unsigned casting instead of truncating the sign bits.
- thick_begin: added clearing of wr_req before starting a new thickness block.