Set seamless cubemap filtering in StateManagerGL. 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/296966 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>