Starmath has a little timer that it uses to trigger the re-render of the
equation. But this timer always triggers itself to run again, so even if the
equation content hasn't changed it always fires. If we only trigger it when
there has been some event that requires it (say UI events or SetText) then we
save battery life by avoiding cpu wakeups.

How about the following trivial patch. AFAICS we already set the timeout on key
press and settext. (patches reduces the timeout time while we're at it as
rendering takes very little time on modern machines so makes starmath seem more
snappy, but that's not an important part)