Gameplay

I added some minor features to make the optics game playable and demo-able on Tuesday. These are : 
- a playable level with a maze of walls which can't be moved or rotated
- a laser on the top left that can't be moved but can be rotated
- a prism as target on the lower right part of the board, which can't be moved or rotated
- 4 mirrors in the box, the minimal amount necessary to reach the prism
- a decreased laser attenuation to make the light path visible during the demo on Tuesday