[AD] Thread safety of display code

[Evert Glebbeek <eglebbk@xxxxxxxxxx>]

Couldn't come up with a neat compact subject line, sorry.
I checked whether I should put the code that manipulates the list of  
bitmaps associated with a display in the OS X implementation of  
al_destroy_display() between mutices (mutexes?), but noticed that none  
of the other code that manipulate these seems to do that.
The question is: should it? Or do we just tell people not to try to  
manipulate the same display from multiple threads?
Even if we do that, Allegro may do it if another display is destroyed  
and its bitmaps are transferred to the display the user is manipulating.

Evert