Add Display.top() + left(), useful for combining multi-monitor captures

[Nebual]

I wanted to take a screenshot of all my displays as one large image, but found I needed the relative positioning of the displays to arrange the resulting imagebuffers correctly.

top() and left() return i32's, where the first monitor is commonly at 0,0, and a second monitor positioned to the right and slightly up might be 1920,-100.
bottom() and right() are convenience methods, which should be equivalent to top() + height(), and thus aren't strictly required but are useful.

I have tested this on Windows and Linux, but haven't tried the Mac version (though it compiles on Appveyor at least). This is my first experience with FFI, so there might be a simpler approach to the return struct of CGDisplayBounds...

[Ameobea]

This would be awesome to have! One thing that my custom screenshot application built with scrap was lacking is multi-screen support. Thanks so much for this.