This CL moves the varying packing from the D3D layer up to Program.
This is necessary for WebGL validation, and gives us consistency
for the various back-ends. There may be some additional cleanup work
on the VaryingPacking class, because it does some work that is D3D-
specific.

WebGL requires strict varying packing. Instead of allowing success
unconditionally, it's an explicit error to succeed to pack a set of
varyings that the sample algorithm would fail to pack.

Introduce a new packing mode option to the varying packing class to
handle this different packing style, while keeping our old more
relaxed packing method for ES code.

BUG=angleproject:1675

Change-Id: I674ae685ba573cc2ad7d9dfb7441efa8cb2d55fc
Reviewed-on: https://chromium-review.googlesource.com/423254
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>