Use SDL’s OpenGL headers

SDL/shader.c

@@ -2,7 +2,6 @@
 #include <string.h>
 #include "shader.h"
 #include "utils.h"
-#include <OpenGL/gl3.h>
 
 static const char *vertex_shader = "\n\
 #version 150 \n\

SDL/shader.h

@@ -1,6 +1,7 @@
 #ifndef shader_h
 #define shader_h
-#include <OpenGL/gl3.h>
+#define GL_GLEXT_PROTOTYPES
+#include <SDL2/SDL_opengl.h>
 #include <stdbool.h>
 
 typedef struct shader_s {