FIX: [windows] When generating "\\?\..." filenames, be sure to remove double backslashes, as this leads to an invalid filename

From the win doc: "For file I/O, the "\\?\" prefix to a path string tells the Windows APIs to disable all string parsing and to send the string that follows it straight to the file system"

This popped up when trying to play a filesystem-based bluray. 
libblueray generates paths like "v:\whatever\/BDMV/index.bdmv", leading to "\\?\v:\whatever\\BDMV\index.bdmv".
Windows fails to open the file with an "invalid syntax" error due to double backslash.