Move all packing related logic from Program to VaryingPacking.
Do linkValidateTransformFeedback check first to simplify packing.
Make getPackedVaryings collect all required VaryingPackings in one
run pass, so that packUserVaryings can focus on packing merely.

BUG=angleproject:2241
Change-Id: I5e84e6e4688c80ba7a77a265b8ffaf79f4ddbb87
Reviewed-on: https://chromium-review.googlesource.com/764821Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>