gathers aren't necessarily varying

I noticed is_always_varying() is a little wrong, and this new test demos
how.  This isn't terribly important: in most practical situations
gathers will indeed be varying.

Change-Id: I456d4c7287147726c49ebb5af5af347c65cd21d4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/350602
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed