Nested structs are handled similarly as to non-nested samplers in
structs. They are extracted and named according to the same
pattern.

Also enables functional.shaders.random.all_features.fragment*

The remaining work in the samplers-in-structs implementation is to
translate function arguments.

Bug: angleproject:2494
Bug: angleproject:2595
Change-Id: If8170feb71137d4036d352b2b0078518647d48a1
Reviewed-on: https://chromium-review.googlesource.com/1101569
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>