Move to using pointers in simplifyDraw*

Move to using pointers into buffers. This will allow
multiple runs from blobs to share buffers.

Change-Id: I6ebed2e490c5fe71077ddc921ead145ce17c5ebd
Reviewed-on: https://skia-review.googlesource.com/141049
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
6 files changed