Add a profiling export mode

The new mode encapsulates Generate Release (ideally the slowest export
mode) inside a cProfile call. It then dumps human-readable stats so that
users can profile their exports and send them to a developer.
Improve texture loading slightly

We now load textures using PIL from their locations on the disk. If they
are packed, we unpack them into a temporary file. Future directions
include further reducing the scope of cStringIO usage in the texture
processor.
Slightly less cStringIO...
Let's do alpha multiplying using PIL
Unroll calls to tDxtImage.distance
More pythonic tDxtImage export loops
Small improvements to the drawable writers
Reserve some lists in rgba2texel
Cleanup math in rgba2texel