Investigate window resizing behavior on desktop with Qt Quick on QRhi with all backends

Animations are running too fast during resize with threaded render loop with Vulkan but not GL (on Linux only?). Not really fatal, but not nice and not how things work with non-RHI. Should be investigated and fixed if possible.

This involves testing to ensure window resizing works in a sane way on Windows, Linux, and macOS, with all backends that are applicable to the given platform, while trying both the 'basic' and the 'threaded' render loops.