Wow. This is amazing. So the purpose here is to merge the Xt loop with
       the GTK loop. We don't want to poll the input sources, but apparently
       there's no way to avoid it. So instead disable the poll when there are
       no input sources attached to Xt. But you can't figure that out.

       Somewhere in this struct, we know there is a field for how many
       input sources there are, but depending on compiler flags, etc., we
       don't know where they are. So we pick a range of offsets it may be
       in and start probing: before there are any input sources registered
       (I don't think it actually guarantees this), we add one to
       /dev/null. Now gather all offsets of shorts which contain a 1. Now
       add another one and filter the list down to all the ones that have
       gone up to 2. Repeat this up to 4. If we are down to one offset,
       save that offset and check there for the input count. Otherwise fall
       back to always polling.