Support trailing ; in filter_complex expression

[Jay Dolan]

Summary of the bug:

It'd be nice if the complex filter parser didn't bomb out with a cryptic error message when a user terminates their filter chains with a trailing ;.

How to reproduce:

% ffmpeg -i "$1" -i "$2" -i "$3" -i "$4" -filter_complex "
    [0:0] pad='max(240\,iw):max(180\,ih):0:0', scale='240:180', pad='480:180:0:0' [left_video];
    [1:0] pad='max(240\,iw):max(180\,ih):0:0', scale='240:180', pad='480:180:240:0' [right_video];
    [left_video] [right_video] overlay;
    
    ...
        
    [0:1] [1:1] amerge;
    " -acodec libfaac -ac 2 -vcodec libx264 -r 15 "$5"

...

[AVFilterGraph @ 0x7fb1cac2c1a0] No such filter: ''

[Carl Eugen Hoyos]

Does it work with current git head?

Please add a minimal failing command line together with the complete, uncut console output to make this a valid ticket.

[Clément Bœsch]

We should indeed support those "empty statements".