Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=53869
Performance improvement for pages with lots of heavily nested tags. Retain a reference to the root JSP context rather than traversing the hierarchy on every call.
Based on a patch suggested by Sheldon Shao.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1438464 13f79535-47bb-0310-9956-ffa450edef68