Support HiDPI displays on vanilla emacs

Fixes the long-standing issue of displaying pdf contents
in proper resolution on retina Macs using vanilla (not a fork) emacs
[1], [2]

This commit squashes changes contributed by @iostapyshyn. It
introduces the following changes:

- Use `frame-scale-factor` to detect HiDPI displays in preference to
  other methods. `frame-scale-factor` is a new function introduced in
  Vanilla Emacs to return the scale factor of the backing store.
- Allow scaling for `png` type images as well (if
  `pdf-view-use-scaling` is `t`), since this works well and I'm not
  sure why it was originally disabled.

1: politza/pdf-tools#51
2: hlissner/doom-emacs#4989

Closes #13

1 files changed, 7 insertions, 8 deletions

diff --git a/lisp/pdf-util.el b/lisp/pdf-util.el
index 3c4d77b..c57f6ec 100644
--- a/lisp/pdf-util.el
+++ b/lisp/pdf-util.el
@@ -945,15 +945,14 @@ See also `regexp-quote'."
 
 (defun pdf-util-frame-scale-factor ()
   "Return the frame scale factor depending on the image type used for display.
-When `pdf-view-use-scaling' is non-nil and imagemagick or
-image-io are used as the image type for display, return the
-backing-scale-factor of the frame if available. If a
-backing-scale-factor attribute isn't available, return 2 if the
+When `pdf-view-use-scaling' is non-nil, return the scale factor of the frame
+if available. If the scale factor isn't available, return 2 if the
 frame's PPI is larger than 180. Otherwise, return 1."
-  (if (and pdf-view-use-scaling
-           (memq (pdf-view-image-type) '(imagemagick image-io))
-           (fboundp 'frame-monitor-attributes))
-      (or (cdr (assq 'backing-scale-factor (frame-monitor-attributes)))
+  (if pdf-view-use-scaling
+      (or (and (fboundp 'frame-scale-factor)
+               (truncate (frame-scale-factor)))
+          (and (fboundp 'frame-monitor-attributes)
+               (cdr (assq 'backing-scale-factor (frame-monitor-attributes))))
           (if (>= (pdf-util-frame-ppi) 180)
               2
             1))