[Bf-cycles] Factor input for Add shader

[Gottfried Hofmann]

On 10/26/2012 10:20 PM, Lukas Tönne wrote:
> Just be aware that the resulting lighting will not be physically
> correct, more like a weird emission shader.

Note that the existing "Add"-Shader suffers from excatly the same 
problem but it's still there for all the NPR- and other arty guys who 
don't need or want physically correct shading.

My proposal just extends the existing Add-Shader with more control...

Cheers,
Gottfried