[AD] Patch for deadlock problems in OSX port

[Valley Way <valleyway@xxxxxxxxxx>]

As originally reported by Ultio, confirmed by Matthew Leverton, and 
discussed in this a.cc post:
http://www.allegro.cc/forums/view_thread.php?_id=498239

Please find attached a patch to:
1. Moved update_dirty_lines from the event mutex
 2. Changed the acquire/release code so that the bitmap->id field is 
updated correctly (only on first acquire/last release)
 3. Changed to thread-safe mutexes implemented by uthreads.c
This hopefully fixes
1. Occasional deadlock on start-up (grabber)
2. Screen locks up when the mouse is moved at the same time as the 
screen is being updated (test program)

I have confirmed that it compiled without errors/warnings on my 10.2 
system, I will wait for replies from others...

Pete

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