Samplers are extracted from arrays of struct uniforms similarly as
with non-arrays. They are named according to the variable name,
array element and sampler field name.

Nested structs to come later.

Bug: angleproject:2494
Change-Id: Ie2f5f7bd1f747e73b3c6b505b2b1043cfe1073b5
Reviewed-on: https://chromium-review.googlesource.com/1101568
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>