A test is added that uses the RedGreenGradient shader, which doesn't
include gl_SampleID, gl_SamplePosition or the sample qualifier, but uses
GL_SAMPLE_SHADING to perform per-sample shading.  The samples from the
resulting image are copied to a buffer in compute and verified.

Bug: angleproject:5395
Change-Id: I3567329e4d567d149e4548eb12222b8dd5255bba
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2559266
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Mohan Maiya <m.maiya@samsung.com>