Use double buffering by default on GLX
Use double buffering by default on GLX

servo/glutin,ecoal95/glutin,tomaka/winit,mbrubeck/glutin,tomaka/glutin,ozkriff/glutin,bvssvni/glutin,mitchmindtree/glutin,vberger/glutin,mbrubeck/glutin,nc4rrillo/glutin