Only show #screen-meta (which encloses the panels) when a panel is showing....

Only show #screen-meta (which encloses the panels) when a panel is showing. Otherwise there's a one-pixel gray line that's really aggravating my OCD. see #18197.

git-svn-id: https://core.svn.wordpress.org/trunk@19137 1a063a9b-81f0-0310-95a4-ce76da25c4cd