change windows backend to sdl2

include/render/GL.h

@@ -0,0 +1,3 @@
+#pragma once
+
+#include "glbinding/gl/gl.h"

include/render/gui/TowerGui.h

@@ -8,7 +8,8 @@
 #ifndef RENDER_GUI_TOWERGUI_H_
 #define RENDER_GUI_TOWERGUI_H_
 
-struct GLFWwindow;
+struct SDL_Window;
+typedef void *SDL_GLContext;
 
 namespace td {
 namespace render {
@@ -21,7 +22,7 @@ class Renderer;
 
 namespace TowerGui {
 
-void init(GLFWwindow* window, td::render::Renderer* renderer);
+void init(SDL_Window* window, SDL_GLContext glContext, td::render::Renderer* renderer);
 void render();
 void destroy();