use SkIPoint for positions in GrTextBlob

Instead of using SkPoint to store position information,
we can constrain the position information to be integers.
This will lead to two future improvements.
  * Shrink position information to SkIPoint16
  * Use integer arithmetic instead of floating point
    in vertex calculation.

 I'm interested in feedback on the comment which describes
 the math behind this technique.

Change-Id: I8441bcbcad99d07e6d6d5e1788d1a47d87f22923
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306948
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
6 files changed