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>