Fix page size/origentation runtime configuration

This fixes the previous commit, which was causing an internal inconsistency
between the Canvas and PDF properties of dompdf. We are still incomplete
functionally since we are wiping out the instances of the Canvas and
FontMetrics objects that were created on dompdf instantiation. Any actions
taken on the canvas object prior to rendering (e.g. CPDF page scripts) are
thus wiped out.

We should integrate the code from #613 so that Dompdf/CPDF is able to set
pages sizes dynamically. PDFLib support will have to be similarly updated.