Fix ComboBox drawing over cursor (in mt_ui 1)

[TheLostInPlace]

With mt_ui 1 the ui update runs on the game thread while seqRender processes on the main thread, so the per frame remove and add landed mid iteration and the open list painted after the cursor.

Registering once in the constructor keeps the slot in sorted order and the render callback now draws the list only while the box and every parent are shown.