title: Gstreamer, unable to render local videos using videomixer
body: <p>Using this pipeline I can play two of the same videos using the videomixer, however when I play two different videos it fails.</p>
  
  <pre><code>gst-launch -e videomixer name=mix ! ffmpegcolorspace ! xvimagesink  \
     uridecodebin uri="file:///home/dtolj/projects/test1.mp4" ! decodebin2 ! videorate ! videoscale ! video/x-raw-yuv,width=320,height=180 ! videobox border-alpha=0 top=0 left=0 ! mix. \
     uridecodebin uri="file:///home/dtolj/projects/test2.mp4" ! decodebin2 ! videorate ! videoscale ! video/x-raw-yuv,width=320,height=180 ! videobox border-alpha=0 top=0 left=-320 ! mix. 
  </code></pre>
  
  <p>error:   </p>
  
  <pre><code>ERROR: from element /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin1/GstDecodeBin2:decodebin22/GstQTDemux:qtdemux0: GStreamer encountered a general stream error.
      Additional debug info:
      qtdemux.c(3865): gst_qtdemux_loop (): /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin1/GstDecodeBin2:decodebin22/GstQTDemux:qtdemux0:
      streaming stopped, reason not-negotiated
      ERROR: pipeline doesn't want to preroll.
  </code></pre>