Move SkShadowTessellator to GrShadowTessellator

BUG=skia:6119

Change-Id: I9756b5aeaced1f21a65063470ccb013c7b856f28
Reviewed-on: https://skia-review.googlesource.com/7505
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
5 files changed