Cubemap filtering is supposed to be seamless for ES3+ contexts.

Fixes 298 failures in dEQP-GLES3.functional.texture, mostly in
dEQP-GLES3.functional.texture.filtering and
dEQP-GLES3.functional.texture.shadow.

BUG=angleproject:986
BUG=angleproject:1149

Change-Id: I6054365086bb3b9d9b085c260f88e2c43eb75bfe
Reviewed-on: https://chromium-review.googlesource.com/296966Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>