In gimp_gegl_buffer_set_extent(), clear the full now-out-of-bounds
region of the buffer, instead of only full out-of-bounds tiles;
however, we still make sure to clear full tiles, instead of partial
tiles, as much as possible.  This prevents (parts of) the old
content of the buffer from showing when it's enlarged again.  This
is especially relevant for the image projection, once we add
support for a dynamically-expanding canvas in the following
commits, since the projection of a reexpanded buffer can be
temporarily rendered to the display before it's fully
reconstructed, exposing parts of the old content.