Mouse release event is not fired if focus is lost during a click

[Zakru]

This issue came up while developing #642 (see thread).

If the user presses their mouse down on the window (e.g. minimap) and moves their cursor out of the window/over another window and changes focus (i.e. Alt+Tab), followed by releasing the mouse, the mouse release event will not be received. This results in the game thinking the mouse button is still down (in the minimap example, moving the mouse over the minimap still moves the camera).

[Indy2222]

This is somewhat related to #648.