Split pixelBuffer from pack/unpack state This will refactor will help use packed enums for buffer targets. BUG=angleproject:2169 Change-Id: Ie7ed3e105f89457c67027e6598d7e29503ad355c Reviewed-on: https://chromium-review.googlesource.com/745181 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>