Fixing Text.updateSize when using the VMLRenderer and display === ‘html’

This is applying the change suggested in my https://github.com/jsxgraph/jsxgraph/issues/64#issuecomment-28541778 after discovering [JXG.getDimensions](https://github.com/Learnosity/jsxgraph/blob/5d4073e/src/utils/env.js#L251-L253) has been using offsetWidth and offsetHeight for over 3 years.