[Piglit] refactor and optimize OpenGL test generators

[Matt Turner]

I looked through the patches touching the generators I wrote or
modified. They seem fine, I guess. I'm not really able to judge what's
good Python. The flip side of that comment is that with these patches
applied I'm not totally sure I'd be able to make meaningful changes to
the generators. I guess that's not a problem, since I don't expect
we'll need to.

I sent some comments about places where I think we're being too
pedantic about python style rules.

Patches 15-21, 24-29, and 39-45 are

Acked-by: Matt Turner <mattst88 at gmail.com>