X Border Attributes

XComposite, which is currently our only capture method for windows, doesn't handle X border width. This works in a lot of cases but anytime there's a border, a bug occurs where the lower right is clipped. 
This patch has two goals: 

1) To position and size the capture texture correctly in accordance to the border.
2) Adds a configuration option to allow people to toggle it (which in most cases will simply do nothing) with a default of not including it. 

Connected bugs: https://obsproject.com/mantis/view.php?id=62
It's from before I had a working patch.
Really, that variable can be removed entirely as it never changes from GS_RGBA at this moment. 
Either way, it doesn't effect anything.