* app/gegl/gimpoperationpointlayermode.c
(gimp_operation_point_layer_mode_process): Implemented Erase, Anti
Erase, Color Erase and Replace. These are not normal layer modes
and handle alpha in their own way. In addition to this, the
behavior of Replace doesn't map very well to GEGL which uses
infinite sized "layers".

Completely works the same:
 o Erase
 o Anti Erase
 o Color Erase

Works different but similar:
 o Replace

* app/paint-funcs/paint-funcs.[ch]: Expose
paint_funcs_color_erase_helper() so it can be used in the
GimpOperationPointLayerMode implementation. Once the migration is
complete this function can be moved entirely to the op and be
tailored to work on premultiplied data.

svn path=/trunk/; revision=27502