glstate: Remove bogus assertion.

Meant only for development.
glstate: Handle more than 32 texture units without segfaulting.

Khronos headers only define symbols up to GL_TEXTURE31, but some
implementations support more.

This was a long standing issue, which got recently exposed by a
dumpActiveTexture change in commit
8374a0d66ca9228269e860089c1535fcda9f9c5f.

Fixes #304.