Change melting point for snow with wrong ctype back to 273.15
FRZZ and FRZW changes/fixes

Fix FRZW low temperature transition (accidentally turned into a high temperature transition in b661418d).
FRZZ now melts into FRZW, so that single pixels "form ice that always cools" as in FRZZ description.
ICE update function: was meant to be checking not setting parts[i].ctype. However, turning FRZZ into self-cooling ice seems a logical thing to do, so set ctype of parts[r>>8].ctype, and make all types of ice cause this reaction (not just ICE(FRZW) as was apparently the original intention of the ctype check).