snapvector.c -> .cpp, remove win_shared SnapVector

* Convert snapvector.c to C++ as snapvector.cpp.
* Remove Sys_SnapVector from win_shared.cpp, as it doesn't care about the
  rounding mode and that might introduce behavior differences between platforms.
Merge pull request #243 from RavuAlHemio/noasm

snapvector.c -> .cpp, remove win_shared SnapVector