Make the GrDirectContext a thing and move it to include/private

This is somewhat of a departure from the original context refactoring
plan (i.e., keep GrLegacyDirectContext hidden and switch GrContext over
to be the GrDirectContext at some point). Having a GrDirectContext
earlier will allow us to change some important signatures earlier
(e.g., asDirectContext) and, hopefully, clarify some of the confusion
about the context class hierarchy.

Additionally, this will let us make onGpuSetup take a direct context -
clarifying its purpose vis a vis onDraw (which now takes a recording
context).

Change-Id: I8298a0649bc95843d20bee33ba7fe1d7e73bb839
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299768
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed