qpaeq: fix: resizing on HiDPI

On high-dpi setups resizing qpaeq caused infinite recalculations because event.oldSize().width() returned irrelevant values that resulted in "thousands of sliders desired". Using different approach that may be not as accurate, but works on HiDPI setups without issues.

Long-running continuous resizing caused recalculations to be done for each event that was not really necessary.