mesa: use loop in pop_texture_group() to restore 4 combiner terms
mesa: use loop in pop_texture_group() to restore 4 combiner terms

There's four combiner terms (not 3) with GL_NV_texture_env_combine4.
Use a loop to make the code a little more compact.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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