Add a gl::AttribArray helper type. This is a generic std::array sized to gl::MAX_VERTEX_ATTRIBS. Bug: angleproject:2264 Change-Id: I788659ad25be5708dbab422ac4a16dff60abf154 Reviewed-on: https://chromium-review.googlesource.com/790750 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>