Vulkan: Add OES_sample_shading extension support Support OES_sample_shading extension if the underlying Vulkan ICD supports sampleRateShading. Bug: angleproject:3587 Tests: dEQP-GLES31.functional.sample_shading.* Change-Id: I1b324c1ad3ea3b2157d3cbe0abcdf7085aa4231b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2444213 Commit-Queue: Mohan Maiya <m.maiya@samsung.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>