fix(virtual-background): Prevent memory leak when desktop share as a virtual background is applied.

react/features/virtual-background/middleware.js

 import { MiddlewareRegistry } from '../base/redux';
 import { getLocalVideoTrack } from '../base/tracks';
 
+import { SET_VIRTUAL_BACKGROUND } from './actionTypes';
 import { localTrackStopped } from './functions';
 
 /**
     const virtualSource = getState()['features/virtual-background'].virtualSource;
     const currentLocalTrack = getLocalVideoTrack(getState()['features/base/tracks']);
 
-    if (virtualSource?.videoType === VIDEO_TYPE.DESKTOP) {
+    if (virtualSource?.videoType === VIDEO_TYPE.DESKTOP && action.type === SET_VIRTUAL_BACKGROUND) {
         localTrackStopped(dispatch, virtualSource, currentLocalTrack?.jitsiTrack);
     }