Multi-Select: Box-Select: fixed an issue using ImGuiMultiSelectFlags_BoxSelect2d mode, where items out of view wouldn't be properly selected. (#7994, #1861, #6518)

Because BoxSelectRectPrev and BoxSelectRectCurr were clamped to scope boundaries, dragging mouse outside of the scope would usually keep one axis unchanged.
Amend 15391762ddbd4ad03c11dc1746f0603c75036edc