PDFBOX-1975: fix ImageIOUtil so that resolution for BMP file is passed properly, add different handling of resolution for PNG and the rest

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1576520 13f79535-47bb-0310-9956-ffa450edef68
PDFBOX-1975: add the helper methods for the unit tests (not used yet)

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1576531 13f79535-47bb-0310-9956-ffa450edef68