Have ShadowTessellators transform path; add SkShadowTessellator base class

BUG=skia:6119

Change-Id: I37639ebab43c9f32f48d2d7dbb8d4619efb9b09e
Reviewed-on: https://skia-review.googlesource.com/8061
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed