Refactored to get direct access to GLCanvas3D in Plater and Preview.

WIP: SLA update delayed until mouse is released.

src/slic3r/GUI/GUI_App.hpp

@@ -139,7 +139,6 @@ public:
     ObjectSettings*     obj_settings();
     ObjectList*         obj_list();
     Plater*             plater();
-    wxGLCanvas*         canvas3D();
     std::vector<ModelObject*> *model_objects();
 
     AppConfig*      app_config{ nullptr };