fix bug in video player:if from=xx:xx:xx is used then bkgrnd flashes

common/lib/xmodule/xmodule/js/src/video/display/video_player.coffee

@@ -45,6 +45,7 @@ class @VideoPlayer extends Subview
       modestbranding: 1
     if @video.start
       @playerVars.start = @video.start
+      @playerVars.wmode = 'window'
     if @video.end
       # work in AS3, not HMLT5. but iframe use AS3
       @playerVars.end = @video.end