nv30: fix some s3tc layout issues

nv30: fix some s3tc layout issues

s3tc layouts are a bit finicky - they're packed, but not swizzled.
Adjust logic to allow for that case:

  - Don't set a uniform pitch for POT-sized compressed textures
  - Adjust define_rect API to be less confused about block sizes
  - Only mark a texture as linear if it has a uniform pitch set

This has been tested to fix xonotic (as well as the s3tc-* piglits)
on nv3x and keeps it working on nv4x.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>