Use a placeholder when atom has no rendered player

This breaks scald_image which does not have a player but use the image field (that field is transcoded, too, because it is an image). So, while it does not have a player, it does not need a placeholder. There are a few solutions:
- scald_image creates a faked (empty) player to avoid this fallback thumbnail.
- scald_image creates a player, and eventually hide the thumbnail by default.

However the bigger problem behind is how the transcoder behaves. Should the Image transcoder transcode an image field? How do we know a file field is an image? A video?

We decided to won't fix the Scald Image bug and provide an update path for former incorrect configuration. Change log created http://drupal.org/node/1890112

It has been fixed with ec756e9.

I've just push ad4ffbd that give a proper player for Scald Image, also fix the problem with thumbnail as describe in the change notice.