gallery.py: Use -b:a and -b:v

gallery.py: Use -b:a and -b:v

Avoiding:

  Please use -b:a or -b:v, -b is ambiguous

I'm not sure when the syntax changed, but this allows us to set the
target audio and video bandwidth independently.  The old -bt option is
gone since ffmpeg's 2c5168a (Remove bitrate tolerance from libx264
interface, 2010-11-11, [1]).

[1]: http://git.videolan.org/?p=ffmpeg.git;a=commit;h=2c5168afed8ddd798dae5909dba85ae3b611f894