ARB_sample_shading: Add test to verify the functionality of gl_SampleMask[]

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
ARB_sample_shading: Add test to verify the functionality of gl_SamplePosition

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
ARB_sample_shading: Add compiler tests for new builtins

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>