Improving memory usage for PDF files loaded by chunks

as described in #5329, this significantly reduces memory usage for large files loaded by chunking
That's needs to be changed. The incomplete first chunk from progressive data (it's often 16-32K) gives us enough information to start first page display and read PDF header -- that why we even bothered to add that as initialData and provided checks in the past. Please incorporate logic here and/or in the prepareBuffer to make this "incomplete" information available.