Added support for derivative map baking, which
is accessable as a dedicated baker type. Works
pretty much the same as displacement map baker,
but gives you derivative map.
In fact, inernally this baker is just a filter
which applies on the result of displacement map.
Both regular and multires baking are supported.
Patch by Morten Mikkelsen and self.
This implements AO baking directly from multi-resolution mesh with much
less memory overhead than regular baker.
Uses rays distribution implementation from Morten Mikkelsen, raycast
is based on RayObject also used by Blender Internal.
Works in single-thread yet, multi-threading would be implemented later.
