Fix: use ImageGenerator factory instead of SkCodec, in case the client
provides generators in some other fashion (other than SkCodec).
This reverts commit 442d424a72.
Change-Id: Ia22f44ef83900413f905fe38570d18a640fca98e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303496
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Reed <reed@google.com>