Until we can support all wrap modes for int textures, use a correct
clamping scheme. This fixes several dEQP GLES3 FBO tests.

This shuffles some of the tests in functional.texture.units since
it hard-codes a different behaviour for all wrap modes, ignoring
the sampler setting.

BUG=angleproject:1244
TEST=dEQP-GLES3.functional.texture.*

Change-Id: Ic7e89a111728dfb18821534996bf5b9a5ad172b6
Reviewed-on: https://chromium-review.googlesource.com/313997
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>