Optimize disposal of t3lib_PageRenderer
t3lib_PageRenderer is currently used in the TYPO3 backend and frontend. There the classes template and tslib_fe inherit from t3lib_PageRenderer. However, in frontend disposal it's not required to have the feature set of the page renderer loaded if a request gets served by the cache and nothing is rendered at all.
Thus, instead of using "tslib_fe extends t3lib_PageRenderer", an aggreation shall be used.
Extension can then access the page renderer e.g. by $TSFE->getPageRenderer()->addJsFile()
(issue imported from #M11985)