Add a "clip" property to GimpCavnasTransformGuides. When set, use
gimp_transform_polygon() for transforming the guides and the
bounding box, so that they're properly clipped.
Add a corresponding "clip-guides" property to
GimpToolTransformGrid, and set it to TRUE in GimpToolHandleGrid, so
that the handle-transform tool's guides are clipped properly.
which removes a lot of code from the transform tool, and reduces the
number of canvas items used for the transform grid from possibly
gazillions to one.
