[skif] Skip saving a layer for image filters that don't use source image

Change-Id: Ice1e50be74cabba8c2d189f8bc627372945615b4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/697476
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed