We were missing the cases for 2D Rect samplers and external OES
samplers. This was causing an assert in the WebGL CTS in Debug.

BUG=angle:550

Change-Id: Ieefeef6ff0f9a486bf7667c7efc60dee766f03ef
Reviewed-on: https://chromium-review.googlesource.com/194122Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>