Windows|ClientWindow|Fixed: Configure a GL viewport immediately when a new GL Canvas is ready

Changing the fullscreen anti-aliasing setting from the Control Panel
resulted in an incorrectly sized GL viewport being used.

ClientWindow now configures an (initial) GL viewport on canvasGLReady
equal to the dimensions of the new Canvas.
Merge branch 'master' of github.com:skyjake/Doomsday-Engine