cdn_ctools_render_alter() warns of undefined index if $info['content'] doesn't exist

Following-on to #2114627: Image URLs in Panels' panes: preg_match_all() expects parameter 2 to be string, array given and #1056646: Rewrite image URLs in Panels' panes, here is a patch that fixes the "Notice: Undefined index: content in cdn_ctools_render_alter()" error you get if your ctools $info array doesn't contain a 'content' key, as is expected in the function.

Though, come to think of it, rather than creating a special carve-out for panels panes (and I love Panels...) what about a text input filter that does this, and you can add it into input filter configs regardless of the display context?