fix: allow overlay mouse interaction outside its bounds

[alex-ds13]

This change is related to this discussion on Discord.

This PR changes the overlay's mouse_interaction so that it can still call this function for all open overlays even if the mouse is not over the overlay bounds. This is useful when implementing any sort of drag ability on a widget on an overlay.

Currently if you try to have a widget like a slider that sets the mouse_interaction to Interaction::Grabbing when dragging the slider, this cursor wouldn't show as grabbing if you moved the mouse outside the overlay bounds while dragging. This doesn't seem to be the expected behavior, at lest not to me. I would expect the slider to keep moving and showing the correct cursor while I'm dragging it even if I move the mouse outside the overlay bounds. This PR fixes this.

The issue was that the mouse_interaction function wasn't even being called on an overlay if the mouse was outside it's borders. And even if it was there was still some code on the user_interface.draw() function that wouldn't let you have the overlay_interaction if the mouse wasn't over its bounds.

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[hecrj]

I just got rid of Overlay::is_over altogether, since mouse_interaction can already be used to encode "hoveredness" thanks to the None and Idle distinction.

Hopefully this didn't break much stuff.

Thanks!