Add PDF_TO_CSS_UNITS to the PixelsPerInch-structure

Rather than re-computing this value in a number of different places throughout the code-base[1], we can expose this in the API via the existing `PixelsPerInch`-structure instead.
There's also been feature requests asking for the old `CSS_UNITS` viewer constant to be made accessible, such that it could be used in third-party implementations.

I suppose that it could be argued that it's somewhat confusing to place a unitless property in `PixelsPerInch`, however given that the `PDF_TO_CSS_UNITS`-property is defined strictly in terms of the existing properties this is hopefully deemed reasonable.

---
[1] These include:
 - The viewer, with the `CSS_UNITS` name.
 - The reference-tests.
 - The display-layer, when rendering images; see PR 13991.

src/display/display_utils.js

@@ -18,6 +18,7 @@ import {
   BaseException,
   isString,
   removeNullCharacters,
+  shadow,
   stringToBytes,
   Util,
   warn,
@@ -35,6 +36,11 @@ const SVG_NS = "http://www.w3.org/2000/svg";
 const PixelsPerInch = {
   CSS: 96.0,
   PDF: 72.0,
+
+  /** @type {number} */
+  get PDF_TO_CSS_UNITS() {
+    return shadow(this, "PDF_TO_CSS_UNITS", this.CSS / this.PDF);
+  },
 };
 
 class DOMCanvasFactory extends BaseCanvasFactory {