Adds a GrGLPathRendering class that wraps the NV_path_rendering
extension and manages its various API versions. It also provides
backup implementations when certain NVpr methods from later API
versions are not present on the current system.

R=bsalomon@google.com, kkinnunen@nvidia.com, markkilgard@gmail.com

Author: cdalton@nvidia.com

Review URL: https://codereview.chromium.org/437473002