Added support for multi-threaded rendering in External GE renderer. Still disabled.

git-svn-id: http://jpcsp.googlecode.com/svn/trunk@3415 4ab978e6-3352-0410-996c-0b42db0051c8
Small optimization in External GE renderer.

git-svn-id: http://jpcsp.googlecode.com/svn/trunk@3416 4ab978e6-3352-0410-996c-0b42db0051c8