aspect: add --video-recenter

This ignores --video-align-{x,y} when the video is smaller than the
window in the respective direction.

After zooming in, panning and zooming out, this is useful to recenter
the video in the window.

Unlike doing this by observing osd-dimensions in a script, this is done
before rerendering, so you don't see the image being rendered uncentered
for an instant after zooming out, before being rerendered centered.

Also update --video-align docs while at it.

This doesn't work well with --video-pan-{x,y} because you can move the
output rectangle far away from the image, and when zooming out pan is
abruptly reset to the center. It doesn't feel like natural like zooming
out after changing --video-align-{x,y}. So this commit doesn't set pan
to 0. Also this leaves a way to move scaled images within the window
even with --video-recenter.