Follow size type discussion for Qt 6 and conclude what to do in QRhi context

We mostly use int for sizes and offsets, following existing Qt patterns. (although there's an occasional quint32 for some offsets here and there)

Follow the Qt 6 discussion (qsizetype, q6sizetype, etc.) and see if we need to do something.

Note that in the graphics context what matters is what the underlying APIs define and use (UINT, NSUInteger, VkDeviceSize, etc.), not C++ or some "standard" library.