Rewrite expose to preserve the depth buffer

Rewrite expose to preserve the depth buffer

Sorted objects are now drawn with depth testing enabled. Unsorted
objects are drawn without depth testing. This mostly affects
GRITS_LEVEL_WORD+n objects.

Objects at other levels such as BACKGROUND, OVERLAY, and HUD should
generally disable sorting so that they are always drawn to the screen.
For those levels, the depth-buffer is set to read-only. This preserves
the depth buffer so  that it can be used later on to get the world-level
depth at a given screen location.