Require clone() be implemented by GrFragmentProcessor subclasses

Change-Id: I66ba0978e5748806d563ff4f26000e4e0095ed24
Reviewed-on: https://skia-review.googlesource.com/29042
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
9 files changed