move homogenous with stride to matrixpriv

this appears to be needed only by Skia
internally, so move it out of the public
includes.

R=bsalomon@google.com
Bug: skia:6898
Change-Id: Iebdda8f2c9a8fd953dd44bac9b74158d7491c21a
Reviewed-on: https://skia-review.googlesource.com/85961
Commit-Queue: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
4 files changed