Almost remove GrContext

This is everything except for literally removing the class.

Change-Id: I2f16caf865d1bcf9c0f267aed73313c0676a73bb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/327222
Commit-Queue: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
25 files changed