Ddraw and d3d9 already take care of this.

This prevents dragging a window's title bar behind a menu bar across the top
of a screen, for example.

If the target rect is outside a monitor rect but is between its extremes in
one dimension, that dimension should contribute 0 to the distance, rather than
some arbitrary amount.

Tests show this is just wrong. This patch fixes a regression introduced by
commit 74e3f516.