Factor out the region-chunking logic of GimpProjection into a new
GimpChunkIterator type, providing a generic mechanism for iterating
over a cairo region in discrete chunks.  The iterator doesn't
perform any processing itself, but rather dispenses rectangular
chunks, which the user then processes.

Iteration is broken into intervals, the duration of which is
configurable.  Each iteration begins with a call to
gimp_chunk_iterator_next(), after which
gimp_chunk_iterator_get_rect() should be called in succession to
fetch a rectangle to process, until it returns FALSE, which marks
the end of the iteration.  Updates to the UI should take place in
the interval between iterations, but not during an iteration.  The
iterator dynamically adjusts the chunk size according to processing
speed, in order to match the target iteration interval.

The iterator can be given a priority rectangle, which is processed
before the rest of the region.  It can also be given a
representative tile rectangle, defining a regular tile grid;
dispensed chunks are aligned to the tile grid as much as possible.