Implement multi-sample antialiasing on OS X.

Set the necessary attributes to create a sample buffer for the
OpenGL context of the vtkCocoaRenderWindow.  This allows VTK to
do anti-aliased rendering in the window.  However, multisampling
is turned off by default on OS X because it causes rendering
problems on some Mac models.

Change-Id: I0ff8073e38d8a63e637cfe4e7146e2d1327d65eb
Merge topic 'osx-multi-samples' into master

23f6ba79 Implement multi-sample antialiasing on OS X.