orbit: turn about the surface under the pointer

With nothing selected, Shift+MMB orbit pivoted on the surface at the
view centre, so grabbing a small part off-centre flung the model away.
It now pivots on the solid or mesh surface under the pointer where the
drag began, falling back to the view centre and then the camera target
(#1389).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/update/viewport.rs

@@ -1436,9 +1436,13 @@ impl OpenCADStudio {
                         if sel.orbit.pivot.is_none() {
                             let scene = &self.tabs[i].scene;
                             let bounds = scene.active_model_tile_bounds(vp_size.0, vp_size.1);
+                            // The surface under the pointer where the drag
+                            // began, so the model turns about what is grabbed.
+                            let grabbed = iced::Point::new(last.x - bounds.x, last.y - bounds.y);
                             sel.orbit.pivot = Some(
                                 scene
                                     .orbit_pivot()
+                                    .or_else(|| scene.surface_pivot_at(bounds, grabbed))
                                     .or_else(|| scene.view_center_surface_pivot(bounds))
                                     .unwrap_or_else(|| scene.camera.borrow().target),
                             );