Adapt doc-view to latest image-mode changes

Adapt doc-view to latest image-mode changes

The functions image-scroll-down, image-scroll-up, image-next-line, and
image-previous-line now return pixel values instead of multiples of
character height as of commit 9c66b09950.

* lisp/doc-view.el (doc-view-scroll-up-or-next-page):
(doc-view-scroll-down-or-previous-page):
(doc-view-next-line-or-next-page):
(doc-view-previous-line-or-previous-page):
(doc-view-insert-image): Adapt to image-scroll-up/down and
image-next/previous-line now returning pixel values instead of
multiples of character heights.  Fixes bug#37874.