<AWT Dev> [8] Please review fix for 7175183: [macosx] Objective-C exception thrown when switching monitor configuration

[Alexander Zuev]

Hello,

   please review my fix for CR 7175183: [macosx] Objective-C exception 
thrown when switching monitor configuration

   The NPE happens because of the getBackupSurface method call may 
result in the resetting of the current accelerated surface to null. The 
idea of the fix is to move the invalidation of the accelerated surface 
before the getBackupSurface() call.

Bug description is: 
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7175183
Webrev can be found here: 
http://cr.openjdk.java.net/~kizune/7175183/webrev.01

With best regards,
Alex