Use correct width for \fint

Use correct width for \fint

Both QTextLine::naturalTextWidth() and QTextLine::horizontalAdvance()
return the same value for \fint. However, examining esint10.ttf with
fontforge does not reveal any issue with the metrics. The fact that
\fint seems to be the only affected symbol might be due to its code
point, which corresponds to a space, so that maybe Qt makes some
assumptions on the metrics.

As QTextLine::naturalTextWidth() returns the width of the line that is
occupied by text, in the case of a single symbol we can obtain the
same value by using the width of the rectangle bounding the symbol.