Re-factor the DOMSVGFactory to extend an abstract base class

This is first of all consistent with all of the other (similar) factories, and secondly it will also simplify a future addition of a corresponding `NodeSVGFactory` (if that's ever deemed necessary).

src/display/base_factory.js

@@ -136,8 +136,45 @@ class BaseStandardFontDataFactory {
   }
 }
 
+class BaseSVGFactory {
+  constructor() {
+    if (this.constructor === BaseSVGFactory) {
+      unreachable("Cannot initialize BaseSVGFactory.");
+    }
+  }
+
+  create(width, height) {
+    if (width <= 0 || height <= 0) {
+      throw new Error("Invalid SVG dimensions");
+    }
+    const svg = this._createSVG("svg:svg");
+    svg.setAttribute("version", "1.1");
+    svg.setAttribute("width", `${width}px`);
+    svg.setAttribute("height", `${height}px`);
+    svg.setAttribute("preserveAspectRatio", "none");
+    svg.setAttribute("viewBox", `0 0 ${width} ${height}`);
+
+    return svg;
+  }
+
+  createElement(type) {
+    if (typeof type !== "string") {
+      throw new Error("Invalid SVG element type");
+    }
+    return this._createSVG(type);
+  }
+
+  /**
+   * @private
+   */
+  _createSVG(type) {
+    unreachable("Abstract method `_createSVG` called.");
+  }
+}
+
 export {
   BaseCanvasFactory,
   BaseCMapReaderFactory,
   BaseStandardFontDataFactory,
+  BaseSVGFactory,
 };

src/display/display_utils.js

@@ -26,6 +26,7 @@ import {
   BaseCanvasFactory,
   BaseCMapReaderFactory,
   BaseStandardFontDataFactory,
+  BaseSVGFactory,
 } from "./base_factory.js";
 
 const DEFAULT_LINK_REL = "noopener noreferrer nofollow";
@@ -109,25 +110,8 @@ class DOMStandardFontDataFactory extends BaseStandardFontDataFactory {
   }
 }
 
-class DOMSVGFactory {
-  create(width, height) {
-    if (width <= 0 || height <= 0) {
-      throw new Error("Invalid SVG dimensions");
-    }
-    const svg = document.createElementNS(SVG_NS, "svg:svg");
-    svg.setAttribute("version", "1.1");
-    svg.setAttribute("width", width + "px");
-    svg.setAttribute("height", height + "px");
-    svg.setAttribute("preserveAspectRatio", "none");
-    svg.setAttribute("viewBox", "0 0 " + width + " " + height);
-
-    return svg;
-  }
-
-  createElement(type) {
-    if (typeof type !== "string") {
-      throw new Error("Invalid SVG element type");
-    }
+class DOMSVGFactory extends BaseSVGFactory {
+  _createSVG(type) {
     return document.createElementNS(SVG_NS, type);
   }
 }