Fix issue about misunderstood on Starling viewport/stage management

In Starling paradigm, it has no meaning to force starling's stage size on viewport size.

In a resize stage event handler, only viewport must change. In must case, starling stage size is set once and never change. 

Camera lens size must be set according starling's stage size and not it's viewport (which is device dependant).

some resource on viewport and starling's stage sizing
http://wiki.starling-framework.org/manual/multi-resolution_development