If the vertex of the current Shape is being moved, it may be useful to "glue" it
to a vertex of another shape, if the 2 vertices are close together. This patch
implements this. THe sticky-sensitivity is controlled by the dist_stick
variable, which is currently hardcoded but could be made configurable e.g.
through the GUI.
The patch also uses a new pure virtual method called getShapeFromMappingId(),
which is implemented by Source / Dest Canvas subclasses. getCurrentShape() is
now using this function, so it is now a non-virtual function in MapperGLCanvas.
Tats