Fixes issue #286
Allow the user to run dav1d by piping input from stdin.
To do this, specify -i - and the stdin will be piped
Since there are multiple areas where a file is opened and the fact that there is seeking involved, the workaround is to create a temporary file and put everything from stdin to that temporary file. After that, we continue processing in the same way as a file and remove the temporary file at the end.