Initialize _clippedViewPort to avoid null deref on first render (#1148)

_clippedViewPort is declared but never initialized, unlike its sibling
_previousViewPort. It is only assigned inside updateClippedViewPort(),
which updateViewPort() skips calling whenever _viewPort already equals
the default-empty _previousViewPort — which can be true on the very
first frame depending on how the initial viewport is set up. When that
happens, render() dereferences _clippedViewPort.width/height while it
is still null.

Found while porting a unit test suite to openfl/starling (the Haxe
port carries the identical gap), and confirmed here in the original
AS3 source.

Author: mallyskies

starling/src/starling/core/Starling.as

@@ -280,6 +280,7 @@ package starling.core
             _rootClass = rootClass;
             _viewPort = viewPort;
             _previousViewPort = new Rectangle();
+            _clippedViewPort = new Rectangle();
             _stage = new Stage(viewPort.width, viewPort.height, stage.color);
             _nativeOverlay = new Sprite();
             _nativeStage = stage;