In wp_get_themes() under multisite, filter out allowed themes before creating...

In wp_get_themes() under multisite, filter out allowed themes before creating WP_Theme objects, rather than after. search_theme_directories() provides us the stylesheet of each theme which is all we need to do the proper intersection. This results in major performance gains on sites that have large numbers of themes unavailable to them. see #20103.



git-svn-id: https://core.svn.wordpress.org/trunk@20152 1a063a9b-81f0-0310-95a4-ce76da25c4cd