Cleanup text vertex structs

* Rename SDFT3DVertex->Mask3DVertex
* Add ctor to ARGB to make construction interface the same
  as Mask ctor.

Change-Id: I28560fce0423b11d7dac6e3a4c798ffd0f088794
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/291076
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed