Fix display of a math hull inset in a tight inset

Fix display of a math hull inset in a tight inset

This is a kind of hack. This allows InsetMathHull to state that it
needs some elbow room beyond its width, in order to fit the numbering
and/or the left margin (with left alignment), which are outside of the
inset itself.

To this end, InsetMathHull::metrics() sets a value in
MetricsInfo::extrawidth and this value is added later to the width of
the row that contains the inset (when this row is tight or shorter
than the max allowed width).

Fixes bug #12320.