mesa: number of combiner terms to pop depends on GL_NV_texture_env_combine4

mesa: number of combiner terms to pop depends on GL_NV_texture_env_combine4

If GL_NV_texture_env_combine4 is not supported, setting the fourth
combiner term would generate a GL error.
Of course, I noticed this right after committing the previous patch
to use a loop in the first place.  <sigh>

Note that GL_EXT_texture_env_combine is always supported so the first
three combiner terms are always accepted.