Add a SamplerImpl object and implement it for the GL backend. Passes all tests in dEQP-GLES3.functional.samplers BUG=angleproject:1162 Change-Id: I7713031a677aac2b41889a6b4297ab512e184863 Reviewed-on: https://chromium-review.googlesource.com/301582 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>