GL_SAMPLE_COVERAGE_VALUE is implemented by inserting gl_SampleMask
writing logic to fragment shader.

New test added: MultisampleTest.ContentPresevedAfterInterruption.
- Skip on D3D11 (Bug: angleproject:4609)

Bug: angleproject:2634
Change-Id: Ib44daf0baccc36ea320596d81713156047da059c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2281783
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>