move shadows to device virtual

This CL keeps the impl for each device backend in the
utils file for simplicity (shared helpers). Future CLs
may move into their respective impl as they become
more specialized.

Bug: skia:
Change-Id: I97ce6cdcc5106ebf4c84778f943cc32d0b7613c1
Reviewed-on: https://skia-review.googlesource.com/15893
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
17 files changed