Bug#743585: libcogl20 1.18.0-2 breaks pitivi

[Pierre-Louis Bonicoli]

Control: bts affects -1 pitivi

pitivi uses cogl. Because cogl_path methods had been splitted in a
sub-library (see
https://github.com/GNOME/cogl/commit/8aadfd829239534fb4ec8255cdea813d698c5a3f),
version 1.18.0-2 of libcogl20 breaks pitivi:

$ pitivi
Traceback (most recent call last):
  File
"/usr/lib/x86_64-linux-gnu/pitivi/python/pitivi/timeline/elements.py",
line 89, in do_paint
    Cogl.path_round_rectangle(0, 0, self.props.width, self.props.height,
  File "/usr/lib/python2.7/dist-packages/gi/module.py", line 313, in
__getattr__
    return getattr(self._introspection_module, name)
  File "/usr/lib/python2.7/dist-packages/gi/module.py", line 134, in
__getattr__
    self.__name__, name))
AttributeError: 'gi.repository.Cogl' object has no attribute
'path_round_rectangle'

Note that cogl is not used anymore in recent versions of pitivi
(https://git.gnome.org/browse/pitivi/commit/?id=af65ef5317e2b7a0492ef165f2686c47b7011279).

Pierre-Louis