Fixed a bug where spans did not always work in layouts.

The problem was that the span we stored in the multiCellMap was the
"effective" span (i.e.  the given span subtracted with the number of
ignored rows it would span). Later we used that span to distribute its
size across all its cells.  However, since the span now could be smaller
that the given span, we could sometimes fail to distribute to the last
span(s).

[ChangeLog][QtWidgets][layouts] Fixed a bug where spans sometimes didn't
distribute themselves to the last cells they covered.

Change-Id: I7bfbbe721f0ec4398be6a5f234c109ddfec18514
Task-number: QTBUG-43099
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>
(cherry picked from qtbase/c9db6e52bc9bd9731159fcb4e95fca5cba01bc9c)