Instead of storing a vertex format as a struct with the full info,
instead use an enum, and look up the info when we need it. This
saves a lot of constructor initialization time, operator comparison
time, and storage. It also will allow us to look up D3D format info
more quickly.

BUG=angleproject:959

Change-Id: I202fd1ea96981073bc1b5b232b1ec3efa91485cb
Reviewed-on: https://chromium-review.googlesource.com/277289Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>