asset image: performance improvement getDimensions()

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@4169 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
build auto version (3048) ...

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@4170 1f8fe7d8-47f0-464c-8d0a-336f4953ab05