Add a shadow opacity property to Light3D

[Calinou]

This can be used to make shadows translucent for a specific light. Performance seems to be identical from before, both with shadows enabled and disabled. This is a partial reimplementation of the shadow_color property, which had to be removed for optimization reasons.

The light distance fade system also uses this to smoothly fade the shadow when the light fade transition distance is greater than 0.

[Calinou]

I fixed shadow fade, it now works (tested with both OmniLight3D and SpotLight3D).

[reduz]

Seems ok save for the comment I made. Needs rebase.

[Calinou]

Rebased (with suggestion applied) and tested again, it works as expected on all light types.

[akien-mga]

Thanks!