OpenSystemsDevelopment / qpdf

Refactor QPDF_Stream to use stream filter classes to handle supported
stream filters as well.

Turns out unreadCh is much more efficient than seek(-1, SEEK_CUR).
Update comments and code to reflect this.

This is all that ever worked. The test suite was trying to do
something different from ClosedFileInputSource.

This is needed for some compilers on some platforms, notably current
versions of clang.

StreamDataProvider::provideStreamData now has a rich enough API for it
to effectively proxy to pipeStreamData.

This was being saved until we had to break ABI.

Refactor removeUnreferencedResources to prepare for filtering form
XObjects.