Enable OES_sample_variables extension on Vulkan backend. Add support for built-in variables, uniform state and constants - gl_SampleId, gl_SamplePosition, gl_SampleMask, gl_SampleMaskIn - gl_NumSamples - gl_MaxSamples Bug: angleproject:3588 Tests: dEQP-GLES31.functional.shaders.sample_variables.* KHR-GLES31.core.sample_variables.* Change-Id: Idf37c7b4ccb0331dbda7acd6389ff4e1022ba959 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2477907 Commit-Queue: Mohan Maiya <m.maiya@samsung.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
