GL: Support OES_shader_image_atomic Bug: angleproject:5342 Change-Id: I4fc4133b1f5bba001112c0d06764146e449c3314 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2542310 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>