PDFBOX-1915: Shading types 6 and 7 (Coons and tensor-product patch meshes), implemented by Shaola Ren as part of GSoC2014

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1607727 13f79535-47bb-0310-9956-ffa450edef68