whether they need all data in one go or not.
* gegl/operation/gegl-operation-sink.h: added needs_full boolean to
class structure.
* gegl/operation/gegl-operation-sink.c: added
gegl_operation_sink_needs_full query method.
* operations/io/display.c: 
* operations/io/png-save.c:
* operations/io/save-buffer.c:
* operations/io/save-pixbuf.c:
* operations/workshop/ff-save.c: Updated writing operations that need
the full buffer to say so.

svn path=/trunk/; revision=1786