This makes them consistent with the other texture types. Also adds a
test for using texture(sampler2DRect, ...).

BUG=chromium:757974
BUG=angleproject:1650

Change-Id: Ie966da928ae0c83850da1e530e72c0d501909394
Reviewed-on: https://chromium-review.googlesource.com/946675
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>