Move all fixed-timestep render updates to new file RenderFixed.cpp

As part of fixing #464, I'll need to move these pieces of code around
easily. In #220 I just kind of shoved them awkwardly in whatever
fixed function would be last called in the gamestate loop, which I
shouldn't have done as I've now had to make formal fixed-render
functions anyway. Because these fixed functions need to be called
directly before a render function, and I'm fixing the order to put
render functions in their proper place, so I need to be able to move
these around easily, and making them function calls instead of inlined
makes them easier to manipulate.

desktop_version/CMakeLists.txt

@@ -117,6 +117,7 @@ SET(VVV_SRC
 	src/Otherlevel.cpp
 	src/preloader.cpp
 	src/Render.cpp
+	src/RenderFixed.cpp
 	src/Screen.cpp
 	src/Script.cpp
 	src/Scripts.cpp