Allow to specify display to render on for JoglNewtWindow - take 2

This is a follow up of #48.

I successfully used this patch to run JoglNewtWindow without having a default display specified on Linux (that is, the environment variable `DISPLAY` was empty) but instead specifying the display explicitly in the JoglNewtWindow constructor.

It is fully backwards-compatible and only changes behavior if the new constructor is used.

@gouessej Can you review this? Thanks.