Set OpenGL profile in SDL

This ensures compatibility with OS X's 1.5 shader version.

demo/sdl/main.c

@@ -45,6 +45,10 @@ main(void)
 
     /* SDL setup */
     SDL_Init(SDL_INIT_VIDEO|SDL_INIT_TIMER|SDL_INIT_EVENTS);
+    SDL_GL_SetAttribute (SDL_GL_CONTEXT_FLAGS, SDL_GL_CONTEXT_FORWARD_COMPATIBLE_FLAG);
+    SDL_GL_SetAttribute (SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
+    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
+    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
     SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
     win = SDL_CreateWindow("Demo",
         SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,