holomaze: add an `overhead' view, showing the current room's triggers (walls etc).

Since there often are overlapping things in the display (gates, walls,
and zone boundaries), and I haven't figured out how to do what it looks
as though glBlendEquation(GL_MAX) should do, they are displayed with
random prioritization each frame, leading to (deliberate) flickering
among the alternatives when two or more things hit the same pixel.