Work-around some global namespace polution

With GLFW, you're supposed to `#include` your extension loader before
it, but this header made that extremely awkward. The previous "fix"
would cause build errors on Windows when compiling the software
renderer.

src/Backends/GLFW3/Window.h

@@ -1,6 +1,3 @@
 #pragma once
 
-#define GLFW_INCLUDE_NONE
+extern struct GLFWwindow *window;
-#include <GLFW/glfw3.h>
-
-extern GLFWwindow *window;