The implementation of bump maps in POV-Ray is broken (well, it was probably
never right in the first place).

A discussion of the symptoms can be found here:
http://news.povray.org/povray.general/thread/%3Cweb.493114d0114da4434dea3830%40news.povray.org%3E/?mtop=291496

I think I just also discovered *why* the bump mapping doesn't work:

The bump_map() function gets three parameters:

- a point in map space
- a structure containing info about the normal "texture"
- a vector identifying the normal at the point

These parameters, however, cannot account for rotations of the bump map around
the (raw) normal axis. This is irrelevant with classic textures, but vital with
bump maps because it makes a difference if the slope is rotated around this
axis.

So there's a conceptual issue here that needs fixing.

There is a similar problem with UV mapping: It is vital for UV-mapped bumps that
the information, which directions correspond to U and V, are passed from the
object to the further processing algorithm.

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