minor change in how zlib is included in povmscpp.cpp

corresponds to Perforce change 6080.
minor robustness improvement in IFF image reading code

corresponds to Perforce change 6081.
atmospheric fog, rainbows and sky spheres now use C++ style memory allocation instead of POV_MALLOC

corresponds to Perforce change 6082.
eliminated a host of C-style pointer casts, mostly by replacing with C++-style casts

corresponds to Perforce changes 6083, 6084, 6085.