This patch implements the tone map node for the realtime compositor
based on the two papers:

Reinhard, Erik, et al. "Photographic tone reproduction for digital
images." Proceedings of the 29th annual conference on Computer graphics
and interactive techniques. 2002.

Reinhard, Erik, and Kate Devlin. "Dynamic range reduction inspired by
photoreceptor physiology." IEEE transactions on visualization and
computer graphics 11.1 (2005): 13-24.

The original implementation should be revisited later due to apparent
incompatibilities with the reference papers, which makes the operation
less useful.