Size code in GsApp can double-count runtimes

The following discussion from !1351 (merged) should be addressed:

• @mwleeds started a discussion:

  Not your bug since it's not new with this branch, but this logic seems prone to double counting the size of things. Specifically if any of the apps in priv->related have a runtime or related app that is the same as the main app's priv->runtime or the same as one of the already counted apps in priv->related, it seems like the same size would be counted multiple times. Something to look out for.