This patch intends to implement glSampleMaski on OpenGL
backends.
Refers to: https://chromium-review.googlesource.com/c/487603

BUG=angleproject:1592
TEST=dEQP-GLES31.functional.state_query.*.sample_mask*
TEST=dEQP-GLES31.functional.texture.multisample.*.sample_mask*

Change-Id: If5ddd6ab57259593919a482be80fbdbe29f6f54d
Reviewed-on: https://chromium-review.googlesource.com/573727
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>