[AD] Resizable windows on Mac OS X

[Evert Glebbeek <eglebbk@xxxxxxxxxx>]

The attached patch implements resizable windows on MacOS X. The  
ex_resize2 demo works correctly with this patch, although the window  
goes blank while it is being resized. The backbuffer (see comment in  
ex_resize2) is resized correctly.

I do think that the way resizing currently works is different from  
how it is described in the documentation, but I also think it's  
consistent with X11 and Windows.

Some specific things Peter Hull may want to have a look at (if he has  
time):
There is a slightly odd line of code in osxgl.m, namely
[window setContentView: [window contentView]];
As it suggests, this gets the current view object from the window and  
sets it to be the current view object. Without this line I get  
artefacts and the view does not properly fill the window boudaries.  
I'm sure there's a better way to do this but everything else I tried  
didn't work properly.

I made a new ALOpenGLWindow class that derives from NSWindow in order  
to capture the windowWillResize message. Is this the proper way to do  
this sort of thing in Objective C, or is there a better way that  
doesn't require me to make my own derived class?

Also (Peter), if you prefer that I just commit things that seem to  
work for me and don't look too clunky, let me know.

Evert