This adds some coverage for samplers that's missing from dEQP.
The test shader passes a sampler to an user-defined function, samples
the texture there and sets the returned sample to gl_FragColor. The
test then checks that the shader result is correct.
BUG=angleproject:1261
TEST=angle_end2end_tests
Change-Id: I2cc511e591a4af01515c6fb38fd43c5f15337bcb
Reviewed-on: https://chromium-review.googlesource.com/321622Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
