[AD] (5.1) Mac OSX Patch - added a way to get NSWindow

[Dennis <dgooden@xxxxxxxxxx>]

I've added a way to get the native NSWindow handle for OSX, just like
getting a HWND under Windows, UIView under iPhone, etc.

This adds a new file called allegro_osx.h. I think I got the CMAKE
stuff right, I just copied what it does for allegro_windows.h. It
seems to build and work OK for me.

I made this patch because if you use multiple windows (created with
al_create_display) in your program, there isn't an easy way to get
access to the different native NSWindow's. It's possible, but it's a
pain. With just a single window, it's not as much of a problem.

Patch is attached, let me know if you have any questions/comments.

- Dennis Gooden
dgooden@xxxxxxxxxx

Attachment: getnswindow.patch
Description: Binary data