Added option to smooth out shadows

[Mac-tf2]

Dark shadows look terrible when converted, so I've added an option to fix that. Changing the value of smoothshadows multiplies the lightmap brightness by that number, and makes every lightmapped texture darker by the same amount to compensate for the now brighter shadows.

Before

After

[Panzerhandschuh]

I wonder if there is an automatic way to properly convert the brightness. Maybe check for compile flags in the entity lump to see if there is something that modifies lightmap brightness/smoothness?

[Mac-tf2]

Looking a little more at the problem I think the faces are just too bright by default. mat_fullbright 1 is so much brighter than it should be on world brushes. The darker func_brush faces is how they are supposed to look I'm pretty sure, since those look normal with fullbright on. Hold off on this for now in case a simpler fix is possible in the face lump data.