Support releasing blocks while iterating them

This makes reset() a little easier to follow, and enables more complex
use cases on top of GrBlockAllocator down the road.

Change-Id: Id79d20e2b394248c997259d6d5b5494fc1456acc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/302678
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
3 files changed