Move Pipeline::getNext() and Pipeline::getNextDocument() methods to agg::Pipeline().

This implies replacing their call sites with agg::Pipeline()::getNext() and agg::Pipeline::getNextDocument(), which in turn implies that an instance of agg::Pipeline() is constructed using agg::buildPipeline() function before calling these methods.

This change should be relatively small (affecting few dozens of call sites), because at this moment agg::Pipeline stages and Pipeline document sources are still the same objects.

This ticket also implies removing Pipeline::stitch() methods, which will become redundant.

We should re-run the performance comparison at this point.