Add a texture data fill helper function to tests Several tests need to initialize arrays of RGBA texture data. Add a helper function for this to reduce duplication and to make the code more readable. BUG=angleproject:596 TEST=angle_end2end_tests Change-Id: Id4a35b34a8fad25c2dc263ad8635dd43355a4f17 Reviewed-on: https://chromium-review.googlesource.com/333911 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>