Compute displayed pages first (in PDF).

Compute displayed pages first (in PDF).
(doc-view-current-converter-processes): Rename from
doc-view-current-converter-process.  Update users.
(doc-view-sentinel): Test buffer's liveness.
(doc-view-pdf/ps->png-sentinel): Remove.
(doc-view-start-process): New function.
(doc-view-dvi->pdf, doc-view-pdf/ps->png, doc-view-pdf->txt)
(doc-view-ps->pdf): Use it.
(doc-view-pdf->png-1, doc-view-pdf->png, doc-view-active-pages): New functions.
(doc-view-convert-current-doc, doc-view-goto-page): Use them.
(doc-view-mode): Kill the processes when leaving the mode.