Rendering rethought

After much consideration, I concluded that the particular manner how OGL
handles depth buffer information through its pipeline (or rather: how it
scrambles information pretty badly), makes rendering and compositing in
IOD much more complicated than I had originally anticipated.  Basically,
what happens is that due to a very unfortunate combination of
constraints in the pipeline, including how positions are enforced to be
floats, the clipping mechanism, perspective divide and the entanglement
of w- and z-data, it is not possible to render into 32b fixed precision
depth buffers. Not even remotely, for that matter.

Bottom line, we will have to cope with the traditional 32b float
precision, including the associated depth-buffer precision. Compositing
will rely heavily on individual objects being able to be sliced up into
polygons which sit at a certain depth, etc. While more considerations
will follow over the course of actual IOD development, the issue is
already reflected in how the planet does depth-testing only on a
per-patch basis but uses depth sorting among patches.

The first steps towards depth sorting were now implemented.