Pack VertexAttribType enum. This improves performance slightly in vertex array format checks. Instead of needing to switch on GLenum values we can use packed arrays and tables to determine the values we need. Does not significantly affect performance but will enable future work. Bug: angleproject:3074 Change-Id: I6f4821a463e9b41fe3f8c8967eb3ed4c1d6b84be Reviewed-on: https://chromium-review.googlesource.com/c/1393903 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>