Add extra security check

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@956568 b8457f37-d9ea-0310-8a92-e5e31aec5664
Add crop position param

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@956569 b8457f37-d9ea-0310-8a92-e5e31aec5664
Check capability

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@956570 b8457f37-d9ea-0310-8a92-e5e31aec5664
Check capability

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@956571 b8457f37-d9ea-0310-8a92-e5e31aec5664