Updated Light system.

- Light only defines one method -> uniform(), that setup all the light
settings excepted its position.
- Introduced a sub-trait PositionalLight that defines a position and
allows to setup it via a uniformPosition method.
- Indeed, one often setup a light separately from its position.
- The space argument is optional in uniformPosition, in which case the
position is not transformed.
- Also updated some shaders with « good practices » :-) see Readme.md
in the shader directory.
- Updated WhiteLight, ColoredLight and HemiLight.