tonemap.glsl: Ensure color parameter of tonemap_reinhard() is positive
tonemap.glsl: Ensure color parameter of tonemap_reinhard() is positive

Color values can become negative in the case of negative lights which
leads to undesired behaviour.

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot