Cycles-Bake: Active to select and cage extrusion

New parameters:
* use_selected_to_active=False
* cage_extrusion=0.0

Selected and active are baked using local coordinates.
So they don't need to be in the same location.

Also, the cage always look inwards.

Note, I tested with XNormal and things seem good.
The object normals, however, are coded differently:

Cycles RGB = XNormal R(1-B)G
XNormal RGB = Cycles RB(1-G)

To be investigated ...