st/icon: Ensure icons are updated if theme node is invalid

Icons that are changed while an actor is not mapped may not have a theme node associated with, and thus we'd end up not updating them at all. In fact we return early in st_update_icon(), and this was not an issue until commit 0b1dfbf6, because we'd end up to update the icon anyways once the style was changed (and so with a valid theme node), but since such change we might not updating the icon if no theme detail changed.

To prevent this, add a flag to require an icon update when the theme changed, if no successfully update happened earlier.

This is not needed in other cases of early-return in update_icon (e.g. when the icon size is invalid) because we're already managing such cases elsewhere.

Fixes: #4568 (closed)