Edit tests to reference temporary variables The shader translator is intended to prune unreferenced variables in the future. To maintain test coverage when this is done, most tests that used to have unreferenced GLSL locals and globals are edited to use built-ins or reference the necessary variables instead. BUG=angleproject:2166 TEST=angle_unittests, angle_end2end_tests Change-Id: I3964e7a80d52fc04a95f57e73da6095e433095e8 Reviewed-on: https://chromium-review.googlesource.com/768740 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>