Sort out behaviour of the render() method of TiledMap

[DOSull]

At the moment because TiledMap is a persistent object each time we run render(), whatever the most recent values of its various attributes are set to 'stick' leading to confusing behaviour. For example

# make tiled map
tm.render()
tm.render(use_ellipse = True)
tm.render()

The third call to render() doesn't reproduce the map made by the first call, because the setting change introduced in the second call 'sticks'. This means replacing any configuration attributes not explicitly set in render() with their defaults.

Maybe the current behaviour is preferable, but we need to at least think about what we want.

[DOSull]

Behaviour is fine.