Lighting: move compute_factor in an impl method
Lighting: move compute_factor in an impl method

Factor out a compute_factor method (pun not intended).
This makes the implementation of Filter::render() more readable
and also remove the only part that differs for the diffuse and
specular cases.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg