vf_pad: center image on padded area if negative x/y

vf_pad: center image on padded area if negative x/y

or if x/y go beyond padded area.

This is mostly useful when paired with the aspect option.
Defaults aren't changed.

Idea for this was taken from mpv's soon-to-be-removed expand vf.

Reviewed-by: Paul B Mahol <onemda@gmail.com>