fix drawing performance on Big Sur

[devernay]

I fixed the drawing performance issues on Big Sur in that MacPorts PR: macports/macports-ports#10770

I will try to make a PR here with that same change, but if anybody else has time, please do!

[devernay]

The fix is basically this diff: https://github.com/macports/macports-ports/blob/master/aqua/qt4-mac/files/patch-qt4-contentsformat.diff