vout: translate mouse state from the vout thread

This avoids to lock the display_lock on the main thread (blocking the whole UI thread) to check sys->display and call vout_display_TranslateMouseState() on every mouse event.

The mouse coordinates are generated when the display_lock is unlocked (because it's a UI event), so there is already no guarantee that the coordinates are valid for the current display configuration, so doing it on the main thread did not bring benefits.

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This is a limited independent change that could be merged before !324.

Note that this change alone is not sufficient to fix #25479 (closed), because on 360 video, mouse events then trigger vout_ChangeViewpoint() few milliseconds later, which suffers from starvation before !324.

Side-note: IMO, vout_ChangeViewpoint() should just post a control command (that could be reduced down to 1 even per loop) processed from the vout thread, the caller need not wait until it is actually applied. That way, it would not need to lock display_lock at all.