Fix accidental duplication of Polygon2D start point

Ensures that closure of Polygon2D in the editor takes into account
Grid Snap if enabled. Does this by comparing the polygon start with
grid snap applied to mouse click location. Note: transformation is
applied in order to deal with different editor zoom levels.

(cherry picked from commit b8d55b2)

editor/plugins/abstract_polygon_2d_editor.cpp

@@ -449,7 +449,7 @@ bool AbstractPolygon2DEditor::forward_gui_input(const Ref<InputEvent> &p_event)
 
 					const real_t grab_threshold = EDITOR_GET("editors/poly_editor/point_grab_radius");
 
-					if (!_is_line() && wip.size() > 1 && xform.xform(wip[0]).distance_to(gpoint) < grab_threshold) {
+					if (!_is_line() && wip.size() > 1 && xform.xform(wip[0]).distance_to(xform.xform(cpoint)) < grab_threshold) {
 						//wip closed
 						_wip_close();