Vulkan: Support the single-sampled targets 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/+/2477909 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Mohan Maiya <m.maiya@samsung.com>