Rearrange shader files

The old way just kind of mashed two approaches together.

src/Backends/SDL2/Window.h

@@ -1,5 +0,0 @@
-#pragma once
-
-#include "SDL.h"
-
-extern SDL_Window *window;

src/Backends/Shared/SDL2.h

@@ -2,5 +2,7 @@
 
 #include "SDL.h"
 
+extern SDL_Window *window;
+
 void ControllerBackend_JoystickConnect(Sint32 joystick_id);
 void ControllerBackend_JoystickDisconnect(Sint32 joystick_id);