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/src/editor.h

@@ -285,6 +285,8 @@ void fillboxabs(int x, int y, int x2, int y2, int c);
 #if !defined(NO_EDITOR)
 void editorrender();
 
+void editorrenderfixed();
+
 void editorlogic();
 
 void editorinput();