Code review comment for lp:~adamreichold/qpdfview/track-exposed-tiles

Maybe we should also consider making "TileItem::cancelRender" inline since even after these optimizations, it shows up on top of gprof's list. I strongly dislike this since it exposes the "RenderTask" internals of "TileItem", but then again "tileitem.h" (and hence indirectly "rendertask.h") are included into "tileitem.cpp" and "pageitem.cpp" only.

It seems like a sensible and pratically free optimization, but it also does not seem to noticeably reduce the "stutter" in any way, so I am very much unsure about it.