Add a new gimp:fill-source operation, which can act as a source
node for fill operations, instead of a fill buffer.  The op takes
a GimpFillOptions object, a drawable, and a pattern offset, and
uses gimp_fill_options_create_buffer() to produce its output.

This allows performing the entire fill operation in chunks as a
graph, instead of allocating a full-size fill buffer, which can
can occupy a lot of space for pattern fills.