This enhances the PackedVarying to be either one element of array, or
whole array. Correspondingly the VaryingPacking is expanded to pack it
properly.

BUG=angleproject:1950

Change-Id: I0529d7ac4367d42b2b433410fbf08351412aada8
Reviewed-on: https://chromium-review.googlesource.com/459115
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>