PDFBOX-1915: Optimization of type 6 and 7 shading by Shaola Ren as part of GSoC2014

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1611159 13f79535-47bb-0310-9956-ffa450edef68
PDFBOX-1915: Optimization based on suggestion by Shaola Ren

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1611163 13f79535-47bb-0310-9956-ffa450edef68
PDFBOX-1915: add missing element in toString

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1611167 13f79535-47bb-0310-9956-ffa450edef68