fix(screenshare): Pass _desktopSharingSourceDevice as shareOptions when available.

Fixes an issue when external cam as screensharing source fails on Spot with multi-stream enabled.

react/features/base/conference/middleware.web.js

@@ -186,6 +186,11 @@ async function _toggleScreenSharing({ enabled, audioOnly = false, shareOptions =
 
     if (enable) {
         let tracks;
+        const { _desktopSharingSourceDevice } = state['features/base/config'];
+
+        if (!shareOptions.desktopSharingSources && _desktopSharingSourceDevice) {
+            shareOptions.desktopSharingSourceDevice = _desktopSharingSourceDevice;
+        }
         const options = {
             devices: [ VIDEO_TYPE.DESKTOP ],
             ...shareOptions