Factorized shading code that was common to PDF and PostScript


git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/branches/FOP-2393_gradient-rendering@1609759 13f79535-47bb-0310-9956-ffa450edef68
Moved content of ShadingPattern into Shading


git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/branches/FOP-2393_gradient-rendering@1609760 13f79535-47bb-0310-9956-ffa450edef68