Added MSAA support for OpenGL ES contexts on iOS.

Note that extra steps must be taken when using glReadPixels to read the contents of the main OpenGL ES framebuffer on iOS, if multisampling is used. See the OpenGL ES section of README-ios.md for details.

include/SDL_syswm.h

@@ -232,6 +232,7 @@ struct SDL_SysWMinfo
 #endif
             GLuint framebuffer; /* The GL view's Framebuffer Object. It must be bound when rendering to the screen using GL. */
             GLuint colorbuffer; /* The GL view's color Renderbuffer Object. It must be bound when SDL_GL_SwapWindow is called. */
+            GLuint resolveFramebuffer; /* The Framebuffer Object which holds the resolve color Renderbuffer, when MSAA is used. */
         } uikit;
 #endif
 #if defined(SDL_VIDEO_DRIVER_WAYLAND)