imagick image adapter bugfix: Imagick::getImageAlphaChannel() cannot be used to get the information if any pixel in the image contains an alpha value - this is now checked pixel by pixel

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@4975 1f8fe7d8-47f0-464c-8d0a-336f4953ab05