This previously was D3D-only, but is required for every draw call.
This completes the work of removing the D3D-specific Impl methods
from ProgramImpl.

Also add several regression tests to cover texture and sampler
validation.

Re-land with a fix for duplicate sampler active uniforms.

Re-re-land with a fix for a test comparison warning on Linux.

BUG=angleproject:1123

Change-Id: Iaf7b33861c07b9ceed4bd53ac2f010d35f05df45
Reviewed-on: https://chromium-review.googlesource.com/301712
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>