MultiSelect: Box-Select: Fixed holes when using with clipper (in 1D list.)

Clipper accounts for Selectable() layout oddity as BoxSelect is sensitive to it.
Also tweaked scroll triggering region inward.
Rename ImGuiMultiSelectFlags_NoBoxSelectScroll to ImGuiMultiSelectFlags_BoxSelectNoScroll.
Fixed use with ImGuiMultiSelectFlags_SinglaSelect.
5 files changed