gui: fix zoom-to-mouse

Plater disappeared when zoom-to-mouse enabled and user tries to zoom
with a side view enabled (Ctrl+<3..6>).

Mouse/screen center position estimation in 3D is made by calculating
intersection of a mouse vector which comes 'through' the screen with an
XY plane. However, when screen is parallel to Z axis, intersection point
located at infinite which results in infinite camera translation vector.

This change switches mouse position estimation to use of projection
matrix like its done for camera panning.

src/slic3r/GUI/GLCanvas3D.cpp

@@ -3696,9 +3696,11 @@ void GLCanvas3D::on_mouse_wheel(wxMouseEvent& evt)
     }
     else {
         auto cnv_size = get_canvas_size();
-        auto screen_center_3d_pos = _mouse_to_3d({ cnv_size.get_width() * 0.5, cnv_size.get_height() * 0.5 });
-        auto mouse_3d_pos = _mouse_to_3d({evt.GetX(), evt.GetY()});
+        float z{0.f};
+        auto screen_center_3d_pos = _mouse_to_3d({ cnv_size.get_width() * 0.5, cnv_size.get_height() * 0.5 }, &z);
+        auto mouse_3d_pos = _mouse_to_3d({evt.GetX(), evt.GetY()}, &z);
         Vec3d displacement = mouse_3d_pos - screen_center_3d_pos;
+
         wxGetApp().plater()->get_camera().translate(displacement);
         auto origin_zoom = wxGetApp().plater()->get_camera().get_zoom();
         _update_camera_zoom(delta);