main: Added entry point for BindTextureUnit.

main: Added entry point for BindTextureUnit.

The following preparations were made in texstate.c and texstate.h to
better facilitate the BindTextureUnit function:

Dylan Noblesmith:
mesa: add _mesa_get_tex_unit()
mesa: factor out _mesa_max_tex_unit()
This is about to appear in a lot more places, so
reduce boilerplate copy paste.
add _mesa_get_tex_unit_err() checking getter function
Reduce boilerplate across files.

Laura Ekstrand:
Made note of why BindTextureUnit should throw GL_INVALID_OPERATION if the unit is out of range.
Added assert(unit > 0) to _mesa_get_tex_unit.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>