* gegl/gegl-chant.h: renamed dummy_filler to chant_data, this means
that operations implemented with the chanting framework has access
to a location to store a reference to data in the form of a user
struct that is created on demand and properly cleaned up in finalize.
The pointer should be accesed like the chanted properties like:
o->chant_data.

svn path=/trunk/; revision=1994