Add clone methods to unit test GrFragmentProcessor classes

Also adds testing of copying ImageStorageAccess and ref counts of proxies held by cloned FPs.

Change-Id: Ia23220bf65b4df83d1c874b46d8525cc3540f716
Reviewed-on: https://skia-review.googlesource.com/28004
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed