Convert GrMesh to a struct

Converts GrMesh to a struct and changes the names/semantics of its
fields to be more inline with their GL counterparts. Also renames the
"instancing" feature to "pattern", to avoid ambiguity with hardware
instancing.

Bug: skia:
Change-Id: Ia0999d4f9c83b5dd31f81b9bf4f36ed9abd26286
Reviewed-on: https://skia-review.googlesource.com/15157
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
35 files changed