desktopCapture.getSources() returns a promise which should resolve
when capturing finishes. Internally it creates an instance of
DesktopCapturer which is responsible for resolving or rejecting
the promise.
Between the time DesktopCapturer starts capturing frames and when
it finishes, it's possible for its handle to be GC'd leading to
it never resolving.
These changes pin the instance of DesktopCapturer until it either
finishes or errors.
fixes#25595
