icon-location messes up the text position

Using a vertical `icon-location` (i.e. `top` or `bottom`) messes up the position of the text, centering the whole text vertically.

This works fine until you try to use anything other than `text-alignment=left`

As seen in the screenshot, this happens even when there's no icon.

I've tracked down the issue to right here
https://github.com/emersion/mako/blob/d3941db86e686c31d1d1820cb15a501caed70e18/render.c#L312-L316

Removing those lines would fix the issue, but these were added on the same PR that added the `icon-location` option, so it seems like this could have been a concious decision?

It certainly could be considered a feature, as it centers the text without messing up with the actual text-alignment, I'm thinking it could be separated into it's own option (possibly `content-alignment`?) but that's it's own issue, either way it clearly doesn't belong here, but I'm not sure if removing it could break a few setups.