add SkVertices::Builder

Possible next iterations:
- remove another allocation use the SkData trick to share
  the object and its (trailing) data
- store a bit that tells use to free each pointer, allowing
  the builder to "adopt" some allocations instead of copy.

Larger idea:
- merge with drawPoints to have a single object for both.

BUG=skia:6366

Change-Id: Iec33239aa2ad5d00b36469ca0b88934ddf6f22eb
Reviewed-on: https://skia-review.googlesource.com/9604
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
7 files changed