Converts the given bmap into a UV offsets map, needed for environment
bump mapping. An UV offsets map contains, at each pixel position,
the luminance difference values of the blue channel of the original
bmap in u and v direction.
bmap - bitmap to be converted to uv offsets
0 if the conversion failed
, otherwise the bmap pointer
While DOT3 bump mapping modulates the texture brighness
according to a light vector, environment bump mapping modulates the
texture UV offset, creating bumps on a mirror like surface.