Images may disappear when editing Image styles

This is a continuation from #764782: Port to Drupal 7 and only affects Drupal 7. In Drupal 6, we utilize the fact that ImageCache freely allowed access to image derivatives and generated them on the fly. This is not the case in Drupal 7, which does not allow direct access to derivative file paths because of the dependency on Clean URLs.

Insert module currently is substituting the old ImageCache behavior into Drupal, but not doing it 100% equally. Right now access to create a derivative is only allowed when editing a node, not when viewing it. This means that editing an Image style will flush the derivative images, and then the necessary derived images won't be recreated when the node is viewed.

For a better example of what I'm talking about, this problem can be reproduced with the following:
- Configure the "article" content type and edit the Image field. Enable Insert and an Image style preset as an insert style.
- Create a new article node, insert the image style (such as medium or large).
- View the node, the image should show up fine.
- Visit the Image Styles configuration page, override the image style used and change it somehow.
- View the node again, the image will be a file not found.
- To "fix" the problem, you can click "edit" and then view the node again, now the image should show up fine.