fix: use unsigned int so that 0xFFFFFFFF can be stored

[aminya]

Cherry-picked from #28

• It fixes a bug where 0xFFFFFFFF was being narrowed down to int, which resulted in an error in the Clang compiler. This was fixed by adding unsigned int to the enums:

C:/opennurbs/opennurbs_defines.cpp:2032:8: error: case value evaluates to -1, which cannot be narrowed to type 'unsigned int' [-Wc++11-narrowing]
  case os_all_snaps:     osm = os_all_snaps; break;