Add support for using ffmpeg to transcode on the fly

[cnelson]

If ffmpeg is installed on the host we are on, use it to do file serving and stream conversion

[cnelson]

Sketch:

Use ffprobe to probe the file and see if it is already playable. Code looks something like:

# parse our input file
try:
    obj = json.loads(
        subprocess.check_output([
            'ffprobe'

            "-v", #no output other than json
            "quiet",

            "-print_format", #output in json
            "json",

            "-show_format", #tell us about the container
            "-show_streams", #and whats inside

            path_or_url
        ])
    )
except subprocess.CalledProcessError:
    obj = None

For now, already playable for means:

• Container is 'mov,mp4,m4a,3gp,3g2,mj2'
• Video must be track 0 and be 'h264'
• At least one 'aac' audio track (it's ok to send other tracks, AC3, etc)

In the future, we could use server_info() to inspect the device features and identify what formats it supports.

If the file meets those criteria then we don't need to do anything, play like we do today

If it needs a transcode, create a temporary directory (optionally allow the user to specify the base path, as on many systems, the default path returned by tempfile can be quite small) and launch ffmpeg in a subprocess with options like:

ffmpeg -i path_or_url -hls_list_size 0 -hls_flags single_file /some/generated/tempdir/foo.m3u8

We'll use RangeHTTPServer to deliver the HLS files to the AirPlay device. This will require a small change to RangeHTTPServer so it takes a list of allowed files (currently it only allows one) so it will allow access to both the m3u8 file and the .ts file ffmpeg will create.

Code would look something like:

if ap.can_play(path_or_url):
    url = path_or_url
else:
    # will use atexit to cleanup tempdir, so we don't leave huge video files lying around
    hls_files = ap.transcode(path_or_url, tmpdir=None)

    url = ap.serve(hls_files)

ap.play(url)

[cnelson]

a83f206 fixes RangeHTTPServer to handle serving multiple files.