Don't evaluate onWebcamLoaded more than once

Firefox for some reason keeps triggering the event and we don't want to
do unnecessary work on each trigger (and flood the log with debug
statements).

src/octoprint/static/js/app/viewmodels/control.js

@@ -402,6 +402,8 @@ $(function() {
         };
 
         self.onWebcamLoaded = function() {
+            if (self.webcamLoaded()) return;
+
             log.debug("Webcam stream loaded");
             self.webcamLoaded(true);
             self.webcamError(false);