Update clutter-gst to 2.0.8.
while here, enable introspection optin by default.

clutter-gst 2.0.8 (18/09/2013)
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List of changes since 2.0.6

o Fix a crash in ClutterGstPlayer when disposing instances too fast

o Fix a crash in ClutterGstVideoSink

o Fix a crash in ClutterGstVideoSink when running on Wayland with X11
  backend compiled

Many Thanks to:

    Thomas Wood

clutter-gst 2.0.6 (24/05/2013)
===============================================================================

List of changes since 2.0.4

o Fix a bug when subclassing ClutterGstPlayer class

o Add support of X11 pixmap as video texture when decoding with
  Gstreamer-vaapi

Many Thanks to:

    Gwenole Beauchesne
    Sjoerd Simons

(obache)