fix bug with attachments being stretched

When I added support for stretching models to any arbitrary dimensions I broke the old behavior of stretching the maximum dimension to a specific size which was being used by attachments. This fixes that issue.

There are still a couple problems we may want to look into:
1) When you remove an attachment using hydras (editModels.js) it goes into an arbitrary distorted cube shape.
2) attachments on avatars don't support non-aspect ration dimensions, so if you attach a stretched model to your avatar it goes from being stretched to being "natural" size. -- Supporting stretching of attachments would be a new feature request.