Derivative filtering for multires baker

Hi,

This is just another new function from Morten Mikkelsen. It adds an option to create derivative map when baking displacement map with multires baker.

There are possibly two things which better be changed:
- Naming and description of some of new settings. It's mostly about name and description for multiplier which adjusts derivatives to heights map.
- Way of reporting auto-scale to user. Currently op->reports are used but there's no report visible after baking until you'll move mouse.