This adds some test coverage that's missing from dEQP.

The test uses two different samplers in a shader, a cube map sampler
and a 2D texture sampler, adds the sampling results together and checks
if the result is correct.

BUG=angleproject:1261
TEST=angle_end2end_tests

Change-Id: If21e24c964fc208b5d1ccd446834dabf13194944
Reviewed-on: https://chromium-review.googlesource.com/321621Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>