GEGL operations. Optimized versions can be added with either
gegl_operation_class_add_processor (class, process_fast, "fast");
by setting the environment variable GEGL_QUALITY to fast this
alternate code path will be chosen.
* gegl/operation/gegl-operation-processors.c:
(gegl_operation_class_add_processor): added infrastructure.
* gegl/operation/gegl-operation.h: added function prototype.
* gegl/operation/Makefile.am: added new file.
* operations/common/invert.c: (process_fast): added implementation
with vertical artifacts.
(gegl_chant_class_init): register the fast version.

svn path=/trunk/; revision=2183