Convert GrMesh back to a class

Specific methods that limit the data to valid configurations are better
than a runtime mega-assert.

Bug: skia:
Change-Id: Ie15f2dc79659e44cfaddd16eb474795b110fda73
Reviewed-on: https://skia-review.googlesource.com/16577
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
26 files changed