Vulkan: Change sampleCoverage calculation 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>