Make NVPR a GL context option instead of a GL context

Make NVPR a GL context option instead of a GL context.
This may enable NVPR to be run with command buffer
interface.

No functionality change in DM or nanobench. NVPR can
only be run with normal GL APIs.

BUG=skia:2992

Committed: https://skia.googlesource.com/skia/+/eeebdb538d476c1bfc8b63a946094ca1b505ecd1

Review URL: https://codereview.chromium.org/1448883002
9 files changed