[AD] Two custom mouse cursor patches

[Evert Glebbeek <eglebbk@xxxxxxxxxx>]

Attached are two patches related to custom mouse cursors. I would  
like some feedback before commiting them (and the code can use a bit  
of polish as well as comments too).

The first adds support for custom mouse cursors as well as system  
cursors to OpenGL/MacOS X, at least for windowed mode (I haven't  
tested if the same setup would work in fullscreen mode too).
I implemented this in src/macosx/osxgl.m, at first without realising  
that Peter Hull had already done part of the work in src/macosx/ 
qzmouse.m. It probably belongs more properly in qzmouse.m, although  
this way the display driver vtable can be set up without needing to  
make the functions global. I can change this anyway as part of the  
cleanup if desired. Much of this code was actually almost directly  
copied from Allegro 4.2's code base.
Peter: how does this code look to you? Any traps I might have fallen  
into with this? Other comments?

The second patch adds text to ex_mouse_cursor explaining how to use  
the program. It uses TTF fonts at the moment because those seem to be  
the only ones that work on MacOS X at the moment. Let me know if it's  
ok and I'll commit it.

Evert

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