Change _rect and _bounds to _fillBounds and _lineBounds BoundingBoxes

This is almost completely working: I still have to debug failing tests in `unit/displayObjectContainer.js`.

On the bright side, this already caught a few correctness issues, and will allow us to be much more secure in assuming correctness of the whole Twips vs Pixels thing. And of bounding boxes in general: right now, Matrix#Transform* creates invalid bounding boxes under certain conditions. That's an error that won't be possible anymore.

@yurydelendik, @mbebenita, @tobytailor, @claus: reviews would be appreciated.