point composers with 3 inputs as requested in bug #548007.
* gegl/operation/gegl-operation-composer3.c: new file
* gegl/operation/gegl-operation-composer3.h: new file
* gegl/operation/gegl-operation-point-composer3.c: new file
* gegl/operation/gegl-operation-point-composer3.h: new file
* gegl/operation/Makefile.am: added new files.
Added the new classes to the following places:
* gegl/gegl-chant.h: allow subclassing the new classes with the
chanting framework.
* gegl/gegl-plugin.h: include the new headers.
* gegl/operation/gegl-operation-processors.c: make the type-aware
functions pointer lookup aware of the new classes.

svn path=/trunk/; revision=2580