I just noticed that "filter" does not work properly with (multi-layered) 
sky spheres.

When using "transmit" in the pigments of a multi-layered sky_sphere, 
they behave exactly like layered textures with the corresponding 
pigments, i.e. the following look virtually identical:

(A)
   sky_sphere {
     pigment { Sky_Pigment_1 }
     pigment { Sky_Pigment_2 }
     pigment { Sky_Pigment_3 }
   }
(B)
   sphere { 0, 1
     texture { pigment { Sky_Pigment_1 } finish { ambient 1 diffuse 0 
specular 0 } }
     texture { pigment { Sky_Pigment_2 } finish { ambient 1 diffuse 0 
specular 0 } }
     texture { pigment { Sky_Pigment_3 } finish { ambient 1 diffuse 0 
specular 0 } }
     scale 10000
     no_shadow
   }

However, when using "filter" instead, the layered-texture sphere's look 
changes significantly (as expected), while the sky_sphere still looks as 
if "transmit" had been used (unless the background can be seen through, 
but in that case it still looks different from the layered-texture sphere).

As with many errors I reported here recently, this one appears to have 
been around ever since, so a change would affect existing scenes.

I'd advocate to change it nonetheless (or make it depend on the #version 
statement), to give the same results as the layered-texture sphere.

Comments, anyone?

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