fftools/ffmpeg_filter: add -autoscale to disable/enable the default scale

fftools/ffmpeg_filter: add -autoscale to disable/enable the default scale

Currently, ffmpeg inserts scale filter by default in the filter graph
to force the whole decoded stream to scale into the same size with the
first frame. It's not quite make sense in resolution changing cases if
user wants the rawvideo without any scale.

Using autoscale/noautoscale as an output option to indicate whether auto
inserting the scale filter in the filter graph:
    -noautoscale or -autoscale 0:
    disable the default auto scale filter inserting.

ffmpeg -y -i input.mp4 out1.yuv -noautoscale out2.yuv -autoscale 0 out3.yuv

Update docs.

Suggested-by:  Mark Thompson <sw@jkqxz.net>
Reviewed-by:   Nicolas George <george@nsup.org>
Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
Signed-off-by: Linjie Fu <linjie.fu@intel.com>