render: Rework damage tracking debug mode

Before this change, the damage tracking debug flag enabled a mode that displayed only the updated contents of the screen and filled the rest with solid color. This may be useful when developing wlroots' damage tracking, but it's not very effective for practically anything else, since actually using the device in such mode is close to impossible.

This commit changes it in a way that makes it mark the updated screen areas with semi-transparent color rectangles. Yellow color represents currently updated area, while magenta shows areas that have been updated in a last frame (so single frame changes can still be visually inspected).