Emulated by applying a mask on top of the mask from glSampleMask.

Bug: angleproject:3204
Change-Id: I4c80ab0a3261cbf1b8a43c270fe13c2a247ea663
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1639749Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>