Make vertex structs private

Change-Id: If09866766b4827dd8390094f1481f766f2ea28f7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/294301
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed