Cut out all of the old texture environment code and rewrote it from

Cut out all of the old texture environment code and rewrote it from
scratch.  There were just too many cut-and-paste errors in the code,
and it was too hard to follow.

This fixes Mesa bugzilla #1508.  #1509 is probably also fixed, but
I'd like to do some more testing before I close that particular bug.
Additionally, this cuts almost 4,600 lines of code from the driver.