vf_aspect: support variables in evaluating the aspect ratios

Example: -vf setsar=sar="sar*9/10"

Signed-off-by: Rudolf Polzer <divverent@xonotic.org>
Signed-off-by: Anton Khirnov <anton@khirnov.net>
vf_scale: make the dar variable return the DAR.

Before, it just returned width/height. Correct is width/height*sar.

That way it is consistent with DAR as in probe output and setdar.

Signed-off-by: Rudolf Polzer <divverent@xonotic.org>
Signed-off-by: Anton Khirnov <anton@khirnov.net>