mesa: increase number of texture units to MAX_COMBINED_TEXTURE_IMAGE_UNITS

mesa: increase number of texture units to MAX_COMBINED_TEXTURE_IMAGE_UNITS

We were misinterpretting GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS previously.

It's the number of texture units for which we need to keep state; not
just the total number of texture units addressable by the vertex shader
plus fragment shader.

Since sw Mesa independently supports 16 texture units in vertex shaders
and 16 texture units in fragment shaders, the max combined units is 32.

Note that the docs for glActiveTexture() indicate the max legal unit is
MAX(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS, MAX_TEXTURE_COORDS) - 1.

A new piglit test (texunits.c) tests the various texture unit limits.

I'm pretty sure I've got this all right now, but additional reviews
are welcome...