Use zlib library to implement "flate" streams in PDFViewer

[mgroeber9110]

As far as I can tell, the Appl/PDFViewer/main/stream.goc module of the PDF viewer contains a separate Implementation of the Inflate compression algorithm that is also available in the zlib library.

We may want to check if we can use the library code instead of the PDF-specific implementation, so that all code benefits from common optimization work. With the addition of PNG support, we will have additional reasons to work on the Inflate code, e.g. by trying to imorove memory management for the 32K window.

This may also help other PDF-related issues such as #438.