[Bf-codereview] Derivative filtering for multires baker (issue4837062)

[g.ulairi at gmail.com]

Reviewers: bf-codereview_blender.org,

Description:
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.

Please review this at http://codereview.appspot.com/4837062/

Affected files:
   release/scripts/startup/bl_ui/properties_render.py
   source/blender/editors/object/object_bake.c
   source/blender/makesdna/DNA_scene_types.h
   source/blender/makesrna/intern/rna_scene.c