Make InstancedRendering more opList-splitting friendly

Make InstancedRendering more opList-splitting friendly

This CL splits the InstancedRendering class into an allocator and a per-opList op tracker. This was done because we need to allocate the InstancedRendering ops before we know the final destination opList. The InstancedRendering ops are now still all allocated from the same pool but the tracking and execution is done per opList.

Change-Id: Ieddabb6f85f8f187c5e7373f7f6cb155d69a9685
Reviewed-on: https://skia-review.googlesource.com/13860
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>