w32_common: Track when the mouse leaves the window

Windows doesn't send WM_MOUSELEAVE by default unless you ask it to;
request tracking for leave events when the mouse enters the window (or is
moved).

Tracking is automatically de-activated once the mouse leaves the window,
so we have to re-request it every time the mouse re-enters the window.
Merge remote-tracking branch 'origin/master' into lua_experiment