Scissor rect support for DisplayObjectContainer

Hey Daniel,

I've ported my similar pull request from Sparrow (https://github.com/PrimaryFeather/Sparrow-Framework/pull/1070) to Starling. Basically, this adds a scissorRect getter/setter to DisplayObjectContainer. It shouldn't add any CPU overhead for code that doesn't use it.

(This also fixes a bug in the existing Starling scissor-rect code: RenderSupport.finishQuadBatch() needs to be called when the scissor rect changes)