Don't convert png to jpg when cropping a header. Prevents stomping...

Don't convert png to jpg when cropping a header. Prevents stomping transparency. Props SergeyBiryukov, kovshenin. fixes #20555

git-svn-id: https://core.svn.wordpress.org/trunk@20706 1a063a9b-81f0-0310-95a4-ce76da25c4cd