screenshare: check if the user is still presenter after gDM resolves

If the presenter gets changed while the user kept the getDisplayMedia browser picker open, accepting the getDisplayMedia prompt might get the previous presenter´s client-side state inconsistent as if they were sharing. The screen sharing is a no-op though

This adds a check after gDM resolves to see whether the user is still the presenter; if they aren`t, just clean up the gDM stream and make it a full no op