shader_runner support for micro benchmarks

I think shader_runner could be an easy way to develop
quick micro-benchmarks when working on performance.

I found shader_runner only required a few tweaks to
be usable for this with depth clears.

I'm not suggesting (at least in this series), that
we add any micro benchmark scripts to the tree. Rather
I would just like to make it possible to write such
scripts for shader_runner.

The last patch in this series provides an example
usage, but I don't want that patch to be added to piglit.