[vtkusers] BGR -> RGB conversion

[tom fogal]

So I've gotten a textured sphere working (thanks, Jared!) but it looks
as if vtk is assuming the image is in BGR format when it is actually in
RGB format:

tfogal at thetis data $ file earth.png 
earth.png: PNG image data, 512 x 256, 16-bit/color RGB, non-interlaced

The pipeline is TexturedSphereSource -> PolyDataMapper -> Actor, and
PNGReader -> Texture -> (aforementioned) Actor.

Perhaps that type of image isn't supported? Maybe I should convert(1)
to another format? Or is there just a function call I'm missing
somewhere?

-tom