Perfect frames on Metal: ensure atomicity with regards to drawable size

Must figure out a way to ensure the CAMetalLayer's drawableSize is set once, and only once, per frame (frame as in QRhi::beginFrame() - endFrame()).

The goal of https://codereview.qt-project.org/c/qt/qtbase/+/274571 is to allow the renderer to ensure the drawable size stays constant while preparing and submitting a frame. https://codereview.qt-project.org/c/qt/qtbase/+/274704 follows that change but does not guarantee proper atomicity due to API constraint (surfacePixelSize() is more like a getter)