feat: use pointer-constraints to contraint pointer once mouse is down

[WhyNotHugo]

When selecting a region, I sometimes move the mouse cursor onto another screen by moving past the screen's edge. This is especially common when trying to capture a border or corner of the screen.

The pointer-constraints protocol is usable to prevent this from happening: once the mouse button is pressed, prevent the mouse from moving onto other screens, since doing so produces bogus values.

[emersion]

This is an intentional feature: the selection rectangle can span multiple screens.