switch to SkPoint for the idea of drawingOrigin

Start making distinctions between three different uses
of matrices and origins.

* draw{M|O} - the information provided by the draw command.
* initial{M|O} - the drawing information captured when the
                 GrTextBlob was created.
* current{M|O} - the matrix and origin used to create the
                 data that is in the SubRun's vertices.

Where {M|O} == Matrix or Origin.

Rearrange computeTranslation and calculate_translation to
return vectors instead of taking out parameters.
Remove init() call.

Change-Id: I8644d95a560d7065d48424cafa51eaebe303d8b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/259828
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
6 files changed