If vertex shader declares a varying but doesn't use it, and this varying
is being used in fragment shader, linking will fail because the varying
would be removed by spirv-cross.

Fix: initilize the unused varying with zero(s).

New test: GLSLTest.InactiveVaryingInVertexActiveInFragment.

Bug: angleproject:2634
Change-Id: Ia37acd8eea85d5da4ddbca6457d6cddb1b41662c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2281796
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>