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).

(Cherry-pick abf8342aa94bdf06bb324f6723a6743dd628d5c6)

Another aspect ratio fix try. This leaves the setdar addition at the end
(preferred by people).

(Cherry-pick e7c7b0d000e81d24327602e04d8fed400dbb7193)