make drawMatrix use more explicit

Make the code more consistent with:
* drawMatrix - CTM from canvas
* drawOrigin - x, y from drawTextBlob
* positionMatrix drawMatrix adjusted by drawOrigin

Change-Id: Ia22baa2d229f041b77dc6cde063e25f185b8b64a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/473237
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
5 files changed