Improve connecting link logic (#697)

### Current

- Connections are disconnected the moment a link starts being dragged
- Reseating a connection where it came from creates a new connection
- If the process is somehow interrupted, the links are already gone

### Proposed

- Connection is disconnected after a new connection is made
- Rendering is bypassed for the link segment being moved
- Does nothing if dropping a link exactly where it came from
- Adds early return when trying to connect a node to itself

src/interfaces.ts

@@ -290,6 +290,8 @@ export interface ConnectingLink extends IInputOrOutput {
   pos: Point
   direction?: LinkDirection
   afterRerouteId?: RerouteId
+  /** The link being moved, or `undefined` if creating a new link. */
+  link?: LLink
 }
 
 interface IContextMenuBase {