Switched to the core versions of OpenGL ES framebuffer object and renderbuffer functions.

Both the extension (OES) and the core versions work, but the core versions are more correct for GLES2+ and they're less verbose.

Xcode-iOS/Demos/src/fireworks.c

@@ -387,6 +387,9 @@ main(int argc, char *argv[])
     SDL_GL_SetAttribute(SDL_GL_RETAINED_BACKING, 0);
     SDL_GL_SetAttribute(SDL_GL_ACCELERATED_VISUAL, 1);
 
+    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 1);
+    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1);
+
     /* create main window and renderer */
     window = SDL_CreateWindow(NULL, 0, 0, SCREEN_WIDTH, SCREEN_HEIGHT,
                                 SDL_WINDOW_OPENGL |