Issue was caused by _wstat returning EOVERFLOW error because
of file size didn't fit into stat structure which was using
long datatype.
The idea of this patch is to use _wstat64 and _stat64 structure
which is capable storing 64bit file sizes.
Made it a typedef for stat structure used by BLI_stat function
in order to make code easier to follow and avoid ifdefs all
over the place.
Additionally solved issue with BLI_exists which was wrongly
returning False in cases destination file is larger then 4GB.
973f95fa9d