Avoid metrics computation on resize when width did not change

Avoid metrics computation on resize when width did not change

Entering a math inset triggers a work area reize because the math
toobars appear automatically. However, by default these toolbars are
at the bottom of the screen and their presence does not change the
typesetting of paragraphs. Therefore it is useful to avoid a call to
updateMetrics() in the case where the width of the work area did not
change.

Part of bug #12297.