Split RenderFrame's item list into normal and shadow items.
Add method for auto-deciding how to render RenderPrimitives.
Fix shadow render to count primitives in addition to normal rendering.
Fix missing SignalAspects.xnb.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@236 a9f3cfff-6c41-7446-b0ca-052bd761e6eb