[Piglit] ideas for shader_runner subtesting

[Dave Airlie]

Hi,

so I've written the FP64 test generator that spits out a test for GLSL
1.50 + GL_ARB_gpu_shader_fp64 but also spits out the same tests for
GLSL 4.00

This in theory should make sure that when we do GLSL 4.00 that we have
tests for that path as well. Now I'm sure ARB_gpu_shader5 also has a
bunch of tests that will need to also run in a GLSL 4.00 context to
test we get the GLSL compile enables all correct.

So I was thinking off possibly adding subtests to shader_runner, that
if can add another section to it to run the same shader with a
different set of #version + #extension tags,

Anyone looked into this before, or how did we handle this back in the
GLSL 1.30 / GL3x days?

Dave.