sizegroup: Skip resizes on widgets that have resize queued

sizegroup: Skip resizes on widgets that have resize queued

Widgets that already have a resize queued don't need to walk the whole
parent chain and queue another resize. It's enough to do it once per
resize.

This also means that sizegroups cannot use the shortcut of just
invalidating the first widget in the group anymore. That widget might
already have a resize queued while others don't.