OSL improvements / refactors + time related stuff

- Avoid computing expensive OSL shader globals if they are not needed.
- Added OSL dPdtime and dtime support.
- Added dPdtime (world space point velocity) sample scene.
- Added hook for OSL surfacearea.

World space motion vectors as colors example:

![dpdtime](https://cloud.githubusercontent.com/assets/409013/4191726/d0f235c2-3790-11e4-98ab-982c2ec61084.png)

Do you actually use `m_ray_dtime` somewhere in the `PathVisitor`?