Clean up some GrContext uses in the GMs

This is part of the effort to replace GrContext with the GrDirectContext/GrRecordingContext pair.

It also tries out, a bit, the context naming proposal (i.e., rContext and dContext).

Change-Id: Ib4d9881f820a7f8a8c525eba7448b1015526400c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303627
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed