image adapter imagick: use png32 instead of png24 because this causes some problems with transparency on older imagick versions

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@4826 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
Merge remote-tracking branch 'upstream/master'