Add back door support for GLSL tessellation shaders

Implements tessellation support at the Ganesh level, and adds back
door methods for supplying raw GLSL strings directly to the OpenGL
driver. Adds a new gm to verify tessellation is works in GL.

Change-Id: Idfc285b955cbe5e8e6bf0475be8b518b0cc6ed2c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261196
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
39 files changed