use a single point as DirectMaskSubRun's vertex data

For direct masks, the rectangle passed through the SubRun and the rectangle in the atlas have
the same dimensions. This information is redundant. Instead of passing the glyph origin in pos,
pass the top left corner of the glyph. In onPrepare, use the dimension information to produce the
entire glyph bounding box.

Change-Id: I135d4a144366b39560742797572641e96fe482df
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/307842
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed