VOXEDIT: StampBrush allow to shift the center of the stamp brush volume - to be able to stamp into a corner without leaving the existing volume

[mgerhardy]

currently we are translating the stamp volume always to be at 0,0,0 - but we should just ensure that 0,0,0 is somewhere between mins and maxs.

this would allow us to shift the volume and to leave the node volume with the stamp volume also on the lower region boundaries (this is currently only possible in center mode)