Make sure that items don't exceed the layout-computed boundary

When geometries were rounded to their nearest pixel grid geometry they
sometimes exceeded the area the layout calculated them to cover.
This could often be observed by that the visual spacing got shrunk to a
smaller size than was specified.
For small spacings values the visual spacing could even disappear.

With this fix the visual spacing should never be smaller than the
specified spacings value (but might be bigger due to rounding).

Task-number: QTBUG-36235

Change-Id: I5142c793460e50fd2de397abfd2ea6730f1bd042
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>