The actual algorithm is still the same sick algorithm that was used
before. But instead of iterating the mask row-by-row and filling
it in small spans, we now use one pixel_regions_process() loop to
process the whole mask. Makes a significant difference for large
elliptical selections.

Remove gimp_channel_add_segment() and gimp_channel_sub_segment()
as they are not needed any longer and were responsible for the
bad performance.