gst/gstbuffer.override: the GstBuffer overrides seem to be confused about whether they're mini-objects or a GBoxed, a...

Original commit message from CVS:
* gst/gstbuffer.override:
the GstBuffer overrides seem to be confused about whether they're
mini-objects or a GBoxed, and it makes copy_on_write no actually
return a usable gst.Buffer. Fix up places where GstBuffers are
treated as GBoxed to use pygstminiobject functions.
Makes gst.Buffer('blah').copy_on_write() work.
* testsuite/test_buffer.py:
Add test for copy-on-write writability
* examples/buffer-draw.py:
Add an example of drawing on a GStreamer buffer with cairo
* gst/gstpad.override:
Make function static