Quick-Fix Tools: many small improvements

- New buttons for Reset and Apply.  Apply is helpful for improving
viewport render performance, as non-destructive FX complicate the
rendering pipeline.  (Note that applying the non-destructive FX will
create an Undo point, so there's no risk to doing it; it basically just
converts the changes to standard destructive ones.)
- Better handling of image checkpoints
- Re-enable save button when non-destructive FX are applied
- Add tooltips
- Minor layout adjustments
Rendering pipeline: performance optimizations

- Big performance gains at 100% zoom by detecting rect equality and
switching to BitBlt.

- Big performance gains at > 100% zoom by detecting rect inset and
switching to StretchBlt.

- Hopefully a final fix for the "checkerboard outside image bounds"
problem.  I finally tracked down the calculation providing unpredictable
results, and an explicit Int() cast seems to rectify it on my test
images.