lavf: Don't try to update files atomically with renames on windows

lavf: Don't try to update files atomically with renames on windows

On windows, rename(2) will fail if the target file exists. On
unix this trick is used to make sure that people reading the file
either will get the full previous file, or the full new version
of the file, but no intermediate version.

Signed-off-by: Martin Storsjö <martin@martin.st>