[graphite] Expose capture methods through ContextPriv.

Allows the methods to be used to capture in tests and GMs.

Change-Id: I56c70bd18365607560013c8f6dcf0119c1802970
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/583778
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed