Processor: updates to checkpoint handling

In preparation for on-canvas "quick fix" tools, PD's recently created
checkpoint system for non-destructive change detection is getting some
updates.

- Changing to a new tool will commit non-destructive changes immediately
- If a non-destructive change has been committed, the
evaluateImageCheckpoint() function will now return TRUE, so the caller
knows whether they need to re-sync the interface to reflect any
committed changes.