[FrameSvg] Optimize updateSizes

frame->fooHeight is the same as frame->fooFixedHeight if the border is enabled, we only meddle
with margins, not heights. Thus we can just do the lookup once and re-use the value.

Also store the hintFooMargin instead of doing the same exact lookup once for fixed
and once for non-fixed margins.

Differential Revision: https://phabricator.kde.org/D8135