Fix gamma, add Glad, error if glad is compiled with emscripten

src/main.h

@@ -5,7 +5,7 @@
 */
 
 /* #define DEBUG  */
-/* #define GLAD  */
+#define GLAD 
 
 typedef const void * window_t;
 typedef unsigned int id_t;
@@ -154,3 +154,9 @@ void destroy_texture(id_t texture);
 id_t create_palette_texture(const unsigned char colors[][4], unsigned char n );
 
 quat_t poll_input(window_t window);
+
+#ifdef EMSCRIPTEN 
+#ifdef GLAD
+#error undefine GLAD on src/main.h please
+#endif
+#endif