Set filter region on handle move (even if unset)

Writes out to filter's svg repr when knot moved.
This affects all knots on a filtered object.
Fixes #749

Also moves knots smoothly when filter region unset.
Fixes #478

(cherry picked from commit a2d7f039)

src/knot-holder-entity.cpp

@@ -398,6 +398,20 @@ void FilterKnotHolderEntity::knot_set(Geom::Point const &p, Geom::Point const &o
         if(!filter) return;
         Geom::OptRect orig_bbox = item->visualBounds();
         std::unique_ptr<Geom::Rect> new_bbox(_topleft ? new Geom::Rect(p,orig_bbox->max()) : new Geom::Rect(orig_bbox->min(), p));
+
+        if (!filter->width._set) {
+            filter->width.set(SVGLength::PERCENT, 1.2);
+        }
+        if (!filter->height._set) {
+            filter->height.set(SVGLength::PERCENT, 1.2);
+        }
+        if (!filter->x._set) {
+            filter->x.set(SVGLength::PERCENT, -0.1);
+        }
+        if (!filter->y._set) {
+            filter->y.set(SVGLength::PERCENT, -0.1);
+        }
+
         if(_topleft) {
             float x_a = filter->width.computed;
             float y_a = filter->height.computed;

src/knotholder.cpp

@@ -312,6 +312,11 @@ KnotHolder::knot_ungrabbed_handler(SPKnot *knot, guint state)
             }
         }
 
+        SPFilter *filter = (object->style) ? dynamic_cast<SPFilter *>(object->style->getFilter()) : nullptr;
+        if (filter) {
+            filter->updateRepr();
+        }
+
         unsigned int object_verb = SP_VERB_NONE;
 
         // TODO extract duplicated blocks: