Added end2end tests for GL_EXT_texture_rg Bug: angleproject:3195 Test: ./angle_end2end_tests --gtest_filter='Texture2DRGTest.*' Change-Id: I076af9e0b4ee78623dcc9c4e07b4e1bececd05c7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1681109 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>