[Mesa-dev] Fix for bug #35073

[Eric Anholt]

Here's a proposed fix for bug #35073, but I'd like some review.  From
googling, it looks like we're not the first driver to stumble on this.
Here's the note from the bug reporter:

	The OpenGL spec looks a little unclear to me - it says the
	alpha test "applies only in RGBA mode, and only if the color
	buffer has a fixed-point or floating-point format", but I
	don't see what it wants if there is no color buffer. I think
	the most useful and compatible behaviour would be to apply the
	alpha test in this case and to compute the correct alpha value
	first.

I'm agreeing with the reporter here with this patch series.