More colour related refactoring.

Render engine is now almost entirely unaware of the colour model used (currently still RGB, but a spectral model might be a future option).
Also "unbundled" colour and filter/transmit information in many places, in preparation for full-colour transparency computations.
fix photons bug introduced with commit 'eca922a2819fabbadd82c31e4bfadaa0c425b5b5'
Merge branch 'hotfix/photons_eca922a2' into refactor