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.

BUG=angleproject:1123

Change-Id: I167cbc8f47b4a397d2ae6cc507bdac35168bcacd
Reviewed-on: https://chromium-review.googlesource.com/299400Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>