Fixed calculation of bounding box for Flash backend

The DrawableObjects min and max were computed incorrectly, because they are the result of applying an affine transformation to the min and max of the internal shape; however, a min or max may not be min or max anymore after the transformation. This, in turn, was affecting the scene bounding box and picking. See the picked coordinates computed in the following example before and after this fix.

<X3D width="400px" height="400px">
  <Scene>
	<Viewpoint position='0 0 10'></Viewpoint>
	<transform DEF='rotation_world' rotation='0 1 0 3.14'>
		<transform id='transContours' translation='-1.5 0 0'>
			<Shape DEF='box'>
				<Appearance>
					<Material diffuseColor='0.5 0.5 0.5'></Material>
				</Appearance>
				<Box></Box>
			</Shape>
		</transform>
		<transform id='transContours' translation='1.5 0 0'>
			<Shape USE='box'></Shape>
		</transform>
	</transform>
  </Scene>
</X3D>