Add writePoint3() to SkWriteBuffer interface.

Change-Id: I900a661f346a10dbd8002bfe383d1705138feb50
Reviewed-on: https://skia-review.googlesource.com/c/146401
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
5 files changed