Fixes some mistakes with merc draw modes. The glass in the palace level
no longer writes to the depth buffer (it's "water"):

Also fixes the one-frame flickers when level draw orders change. We
might be able to make this more efficient in the future, but this will
at least fix the frame with nothing drawn.
This should fix a bunch of texture-related issues by generating a table
of overlapping textures and just... adjusting them slightly so they
don't overlap. It's not the most elegant solution in the world, but I
think it's no worse than the existing hard-coded tpage dir stuff.