app: use gimp_transform_polygon() in GimpCanvasTransformGuides
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.
Showing with 192 additions and 99 deletions