GrRenderTargetContext scaffolding

I added the renderTargetContext() method to GrTextTarget so I don't
have to convert the entire call stack over in one CL. In the end,
GrTextTarget and GrRenderTargetContext::TextTarget will be deleted.

Change-Id: I49387737c26b69111050101d9da3380617db2918
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295376
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
5 files changed