Extrude brush: /u causes bugs

[Featherblade]

When use the extrude brush, the use of /u will not undo a 'cross' shape between the two most extreme points of the disc perpendicular to the direction the brush was used.

E.g, if the brush is used on the top face of a bock, any blocks changed in a line going from North to South or East to West from the central block will not undo. This 'cross' left over will be the same height as the vh selected.

This happens regardless of parameters and performers.