Re-land "Move sampler validation to the GL layer." 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>