[Piglit] Feature request: indirect rendering

[Brian Paul]

I happened to find a bug in NVIDIA's driver with indirect GLX rendering.

I was going to write a piglit test for it but found that we have no 
way to request an indirect GLX context in piglit.

We could either:

1. add a PIGLIT_GL_VISUAL_INDIRECT bit/flag
2. add a bol piglit_gl_test_config::indirect field (my preference)

If we can't create an indirect rendering context for a test, report 
PIGLIT_SKIP.

This would have to be plumbed down through waffle also.

I'll go ahead and write some patches if there's no concerns.

-Brian