implement support for fractional scaling (wayland-protocols!149)

For details see https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/149

Only regression I found is that using QRectF for pointer confinement makes the confinement region one logical pixel larger than with QRect, don't know how to properly fix that one yet. The simplest workaround would be to round down the rectangle sizes, but for both Xwayland and this protocol it would likely be desired to make pointer confinement work in real pixels.