Vulkan: Change sampleCoverage calculation
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 <7b4e62ba78563ed0257fccea075421f31db8978d@samsung.com>
Commit-Queue: Mohan Maiya <fbcab303f71178634c736986d06722fc897795f2@samsung.com>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Shahbaz Youssefi <582eeec4f08a2ccd61e3ef52115e6e7c571582de@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle