[Feature Request] Add option to center and/or stick to corner (padding-related)

Hello,

I would love to be able to tell Rio to re-distribute the padding so that the actual content is centered and/or sticks to some given corner.

At this moment, the default is always top-left and the left-over space is on the right and on the bottom. It can be somewhat changed using padding, but it doesn't work well (for what I want to achieve) when changing font size or window size.

If I remember correctly, Alacritty has the option to (only) center the content, which redistributes the gaps so that they are even on all sides.

In my case, I like to have zero padding and stick to bottom-left corner (which I achieved in Wezterm with custom dynamic-padding event handler).

My idea is some option in the config file named align-content (or something like that), where user could put one of center, top-left (default), top, top-right, right, bottom-right, bottom, bottom-left, left.

Anyway, thanks for your work, Rio is awesome!

Start by locating Rio's configuration handling and the terminal layout or rendering entry points that determine padding and the top-left default. Compare the requested alignment choices with the existing padding behavior, then identify how configuration changes are tested. Done means the requested alignment options work across font and window-size changes without altering the default behavior.