mesa: fix texenv generation when num color bufs == 0

mesa: fix texenv generation when num color bufs == 0

Before, if there were no color buffers enabled (with glDrawBuffers(GL_NONE))
when the texenv program was generated, we'd emit writes to OUTPUT[1] but
the OutputsWritten mask was 0.  This inconsistency caused an assertion to
fail later in the Mesa->TGSI translation.

Fixes fd.o bug 28169

NOTE: this is a candidate for the 7.8 branch (and depends on commit
b6b9b17d27c570cc99ae339e595cf2f63ca5e8d7).