The "update" signal on drawable or projection can actually be emitted
many times for a single painting event. Just add new signals ("painted"
on GimpDrawable and "rendered" on GimpProjection) which are emitted once
for a single update (from user point of view), at the end, after actual
rendering is done (i.e. after the various "update" signals).

Also better support the sample merge vs current drawable paths for
bucket fill.

(cherry picked from commit 04726533)