... in gimp_drawable_bucket_fill()

gimp_drawable_apply_buffer() already takes the selection mask into
account.  Intersecting the selection mask with the fill region
prior to that leads to wrong (too low) alpha values, in general,
when partial selection is involved.

This commit eliminates the intersection of the fill region and
selection mask data before the apply_buffer() call, although it
does calculate the intersection of their bounds, to avoid
processing regions that won't be visible anyway.