add virtuals in prep for device-clipping

Build flag available for backends to begin testing their impl.

Need to formalize save/restore, and how to forward these to device but not on picture canvases.

BUG=skia:6214

Change-Id: Ic5c0afba3e8c84fcf124567e63fe2f5880b623e7
Reviewed-on: https://skia-review.googlesource.com/8183
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Brian Salomon <bsalomon@google.com>
4 files changed