When emulating the sample coverage to a mask, truncate
the value.

Test: dEQP-GLES3.functional.multisample.fbo*sample_coverage*
Bug: angleproject:4568
Change-Id: Ie0f9fee7dc7cf08e2289ff24f0fa69f9929c4ae3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2433069
Commit-Queue: Brandon Schade <b.schade@samsung.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>