#1716: Render a placeholder image in wysiwyg.

Media used to take care of this before. If a formatter returned nothing for a wysiwyg, it would generate a placeholder image. Now the default behavior is to generate a link to the file instead. This is useless for iframes, so I had to render something. With that fixed, the rendering issue on node view fixed itself.
Var name change from render to output
Merge pull request #1719 from openscholar/1716-media_embed_render

#1716: Render a placeholder image in wysiwyg.
Only change I made was the var name change suggested by @amataibu