Snapshot improvements.

* Warn when it was not possible to make a snapshot because the difference
between the aspect of the required target size and the required target
aspect itself is too large.
* Reset Feed and Postcard Aspect to Default when making a new snapshot.
* Fixed a rounding error that caused one-pixel size error.
More snapshot UI tweaks.
Snapshot Freeze Time: add Langolier tag for new (invisible) avatars.

If an avatar TP's in while you are in Freeze Time mode, they now have a
visible tag with the tag 'Langolier' in it. After exiting Freeze Time
mode they become visible. Before they would stay invisible.