Add notion of active opList to GrDrawingManager (take 2)

With an incrementally sorted list of opLists we will no longer be able to
assume that the last opList is the active one.

This is pulled out of: https://skia-review.googlesource.com/c/skia/+/150614 (Begin centralizing opList DAG) and also contains some of the non-substantive changes from that CL.

Change-Id: I2d122889e595f7240c9a99e4f0a2ede2d9813112
Reviewed-on: https://skia-review.googlesource.com/151371
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed