In GimpProjection's chunk renderer, when the chunk height changes
in the middle of a row, we need to merge the remainder of the
current render area back into the renderer's update region, and
refetch the remainder of the row as the new render area, so that we
don't miss any unrendered area, or re-render already-rendered area,
due to the change in chunk height.  However, we should previously
fail to verify that the fetched area is, in fact, the remainder of
the current row, which could cause us to render the wrong area,
missing parts of the update region.

Fix this, by breaking up some of the chunk-renderer fucntions into
smaller sub-functions, and using those in order to explicitly set
the new render area to the remainder of the current row when the
chunk height changes.  This also avoids erroneously merging the
unflushed update region of the projection into the renderer's
update region.