Make WindowTextFont thread-safe in relevant areas and make texture-generation delayed. Measuring text no longer needs a GraphicsDevice.
Switch SignallingDebugWindow over to using new font code.
Fix SignallingDebugWindow line position so they're centred on start/end points.
Fix line-spacing in TextFlow window control.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@729 a9f3cfff-6c41-7446-b0ca-052bd761e6eb