convert UTF-8 log back to UTF-16 before print it to windows cmd console

The log facility in libav will convert command line parameters to UTF-8 before processing them, but when printing those converted UTF-8 log to windows cmd.exe console, some charactors(such as chinese charactors in file names) will get displayed as garbage charactors because of the mismatch of ANSI encoding in the user's windows environment and UTF-8. This patch will convert them back to windows Unicode strings before output.