Use SkSpan to clean up GrOnFlushCallbackObject API

Also adjust the OpsTask terminology to the broader RenderTask term.

Change-Id: I8549e74a3e2f6b2caf765103f31243b776823c16
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/332724
Commit-Queue: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
11 files changed