mesa: fix num_draw_buffers==0 in fixed-function fragment program generation

mesa: fix num_draw_buffers==0 in fixed-function fragment program generation

This fixes a problem when glDrawBuffers(GL_NONE).  The fragment program
was writing to color output[0] but OutputsWritten was 0.  That led to a
failed assertion in the Mesa->TGSI translation code.

NOTE: This is a candidate for the 7.9 and 7.10 branches.