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

3 files changed, 11 insertions, 20 deletions

diff --git a/lisp/pdf-cache.el b/lisp/pdf-cache.el
index e9a7ca5..50d7688 100644
--- a/lisp/pdf-cache.el
+++ b/lisp/pdf-cache.el
@@ -380,6 +380,7 @@ See also `pdf-info-renderpage-highlight' and
          (pdf-cache-pagelinks
           (pdf-view-current-page)))))))))
 
+(defvar pdf-view-use-scaling)
 (defun pdf-cache--prefetch-pages (window image-width)
   (when (and (eq window (selected-window))
              (pdf-util-pdf-buffer-p))
@@ -388,7 +389,7 @@ See also `pdf-info-renderpage-highlight' and
                   (pdf-cache-lookup-image
                    page
                    image-width
-                   (if (not (pdf-view-use-scaling-p))
+                   (if (not pdf-view-use-scaling)
                        image-width
                      (* 2 image-width))))
         (setq page (pop pdf-cache--prefetch-pages)))
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))
diff --git a/lisp/pdf-view.el b/lisp/pdf-view.el
index 03ff716..bb42e45 100644
--- a/lisp/pdf-view.el
+++ b/lisp/pdf-view.el
@@ -92,10 +92,7 @@ FIXME: Explain dis-/advantages of imagemagick and png."
 This variable affects both the reuse of higher-resolution images
 as lower-resolution ones by down-scaling the image.  As well as
 the rendering of higher-resolution for high-resolution displays,
-if available.
-
-It has no effect, unless either the imagemagick or image-io
-image-format is available."
+if available."
   :group 'pdf-view
   :type 'boolean)
 
@@ -911,12 +908,6 @@ See also `pdf-view-use-imagemagick'."
         (t
          (error "PNG image supported not compiled into Emacs"))))
 
-(defun pdf-view-use-scaling-p ()
-  "Return t if scaling should be used."
-  (and (memq (pdf-view-image-type)
-             '(imagemagick image-io))
-       pdf-view-use-scaling))
-
 (defmacro pdf-view-create-image (data &rest props)
   ;; TODO: add DATA and PROPS to docstring.
   "Like `create-image', but with set DATA-P and TYPE arguments."
@@ -935,7 +926,7 @@ See also `pdf-view-use-imagemagick'."
   (let* ((size (pdf-view-desired-image-size page window))
          (data (pdf-cache-renderpage
                 page (car size)
-                (if (not (pdf-view-use-scaling-p))
+                (if (not pdf-view-use-scaling)
                     (car size)
                   (* 2 (car size)))))
          (hotspots (pdf-view-apply-hotspot-functions