Sync pixel pack state before glReadPixels in FramebufferGL. Fixes conformance/reading/read-pixels-pack-alignment.html BUG=angleproject:885 Change-Id: I48f1598d9a67f3980d2ff86307eb107ec7506776 Reviewed-on: https://chromium-review.googlesource.com/275691 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>