Fix thumbnails not appearing on MinGW64, was actually a mistake on
MinGW-w64 headers (where _stat is actually _stati64), but since we
recommend a specific compiler build it's OK for now. Also tweaked other
places where _wstat is used.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56116 954f8c5b-7b00-dc11-b283-0030488c597c