(doc-view-dvi->pdf-sentinel, doc-view-pdf/ps->png-sentinel)

(doc-view-dvi->pdf-sentinel, doc-view-pdf/ps->png-sentinel)
(doc-view-pdf->txt-sentinel, doc-view-ps->pdf-sentinel, doc-view-display):
Don't change buffer within a sentinel or timer.
(doc-view-display): Don't try to display before the requested page
is available, unless told to do so explicitly.
(doc-view-pdf/ps->png-sentinel, doc-view-initiate-display):
Force display even if the requested page is not available.