[glean-dev] Updates to ttexcombine.cpp

[Gareth H. <ga...@va...>]

I've added tests for EXT_texture_env_dot3 to this glean test.  I check
the availability extension manually and if it's available do tests with
GL_DOT3_RGB_EXT and GL_DOT3_RGBA_EXT combining.  This test, and the
accompanying Mesa implementation, can be trivially updated to the ARB
extension when it becomes available.  The EXT extension has a constant
scale factor of 4, while the ARB one will likely remove this
restriction.

We have the whole glext.h header file update issue to deal with, of
course.  Allen, I wasn't sure if glwrap.h was the right place for the
two enums, so I've just conditionally defined them at the start of this
file.  Any suggestions of a nicer way of handling this would be
appreciated.

I will commit this sometime soon (very busy at the moment).

-- Gareth