Avoid some full metrics computations related to math previews

Avoid some full metrics computations related to math previews

When entering/leaving a math hull inset, a Update::Force flag was set,
in case the metrics of the inset would change because of a switch
between normal and preview representation.

When entering the inset, this code is now used only when the inset was
in preview mode.

In both cases, Update::Force is replaced with Update::SinglePar.

This requites in Text::dispatch to honor Update::SinglePar when it has
been set by some lfun, even when singleparupdate is false.

Part of bug #12297.