Fix for compiling with 10.6 SDK

Hi,

Currently GLFW does not compile on Mac OS X 10.6 (although it runs). This is because `convertRectToBacking:` returns an `NSRect`, but undefined methods in Objective-C are assumed to return `id`. Unfortunately `id` is not castable to `NSRect`, so the build fails.

This patch fixes this problem using a category. Maybe there's a better way, but this seemed to work. Tested on OS X 10.6 and OS X 10.8.