[Xcb] Funky casts

[Jeremy A. Kolb]

So for some reason I can't wrap my head around this.  Don't laugh!

So I have a XCBGlxDRAWABLE.  XCBGlxMakecurrent takes one of these.  I 
SHOULD just be able to pass in an XCBWINDOW since both are xid types and a 
window can be used as a glx drawable (this is how xlib does it).  However 
I cannot pass this into the function and no amount of casting results in 
compileable code.  Most likely my grasp of c sucks ;)

Jeremy