ARB_sample_shading: Add test to verify new APIs and enums
ARB_sample_shading: Add test to verify the functionality of gl_NumSamples
ARB_sample_shading: Add test to verify the functionality of gl_SampleID
ARB_sample_shading: Add test to verify the functionality of gl_SampleMask[]
ARB_sample_shading: Add test to verify the functionality of gl_SamplePosition
ARB_sample_shading: Add compiler tests for new builtins