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.

BUG=angleproject:1123

Change-Id: Iefef06e7901873c98bf2ba7864efd16a4c6435d3
Reviewed-on: https://chromium-review.googlesource.com/301581
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>