Add a transform matrix to GimpCanvasBoundary and get rid of the whole BoundSeg transform code in boundary.c and gimpbrushcore.c, it was impossible to get this right on that level. Also fix te extents of GimpCanvasBoundary os it leaves no artifacts.
and pass it to all constructors. The GimpDisplayShell is needed because items are going to become more powerful soon.
This can be done much better, but this one kindof works for now.