Fix overflow on non-scrolling widgets

[docelic]

Run e.g. test/widget-scrollable-boxes.cr.
If the main box is created with scrollable: false, then the overflowing children are just shown on the screen instead of being permanently hidden (since they're out of visible area, and there's no way to scroll them).
See whether this is intentional or an unhandled case.
Same issue is present in Blessed.