PDF writer abstraction layer

 PDF::Writer is a generalised API that allows a module that generates PDFs to
 transparently target multiple backends without changing its code. The
 currently supported backends are:
  * PDF::API2
  * PDFlib (versions 3+)
  * Mock
 If both PDF::API2 and pdflib_pl are available, PDF::API2 is preferred.
 If neither is available, a run-time exception will be thrown.