Add a possibility to display the "global" view of the model when using specific camera position

[dphilippon]

When focusing on an agent using the different facets of opengl display, the camera follows the agent and we have a more precise view of the model. However, this precision is done at the expense of the global view. I think it would be interesting to display in a corner (left-bottom one for instance) the global view.

This can be an example (done in GAML with a kind of ugly method but, it reaches its goal).

[AlexisDrogoul]

Reopening this issue for the next round of development on GAMA 2.0