[AD] Center window on correct screen (MacOS X patch)

[Trent Gamblin <trent@xxxxxxxxxx>]

Attachment: winpos.diff
Description: Binary data

How does this look? I didn't really understand the code that was there. Anyway this addresses two minor issues. First, the new display adapter is thread local, so we store it in the main thread like new_display_pos_x/y. The I changed the condition of the user not passing a window position. It's supposed to center the window on the screen you've chosen with al_set_new_display_adapter(), but before it was always centering on the main screen. Evert?

Thanks,
Trent