fixed two yet unreported bugs:
- Iridescence did not properly honor full area diffuse lighting.
- Subsurface scattering did not play nice with non-default brilliance values.

corresponds to Perforce change 6076.
require boost >= 1.38

corresponds to Perforce change 6077.
minor improvements in SSLT relating to the storage of precomputed values

corresponds to Perforce change 6078.
more sophisticated way of pulling in the libtiff headers in povray.cpp without breaking the Windows build

corresponds to Perforce change 6079.