ffmpeg: fix -aspect cli option

ffmpeg: fix -aspect cli option

Redesign the way -aspect option is handled. This is done by making
ffmpeg read the sample aspect ratio set in the corresponding input
stream by default, and overriding it using the value specified by
-aspect.

If the output display aspect ratio is specified with -aspect, it is
set at the end of the filterchain, thus overriding the value set by
filters in the filterchain.

This implementation is more robust, since does not modify the
filterchain description (which was creating potential syntax errors).