Rename Graphics#_rect to #_fillBounds and #_bounds to #_lineBounds

This is part 1 of the grand "we change all the rectangles to internal representations" refactoring.
Add GFX.Geometry.BoundingBox and conversion methods from and to flash.geom.Rectangle

Part 2 of the grand "we change all the rectangles to internal representations" refactoring.
Change internal representation of DisplayObject#scale9Grid from Rectangle to BoundingBox

Part 3 of the grand "we change all the rectangles to internal representations" refactoring.
Change all occurrences of _rect and _bounds Rectangles to _fillBounds and _lineBounds BoundingBoxes

Part 4 of the grand "we change all the rectangles to internal representations" refactoring.

Also adds verification code to BoundingBox, ensuring that all properties are integers and (currently disabled because of bugs in using code) that *Max > *Min.