Add support for gl_SampleMask for single-sampled targets.
When dealing with a single-sampled target, gl_SampleMask
is always be set to 0xFFFFFFFF. And when the target is
single-sampled, sample shading is disabled to enable
Bresenham line rasterization.

Bug: angleproject:3588
Tests: dEQP-GLES31.functional.shaders.sample_variables.
           sample_mask.discard_half_per_pixel.*
           sample_mask.discard_half_per_sample.*
           sample_pos.correctness.*
           sample_mask_in.bit*_per_two_samples.*
           sample_mask.discard_half_per_two_samples.*
           sample_mask.inverse_per_*
Change-Id: Ibb471261b8451ff01fab3dc43f2e965ae2999610
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2477909Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>