Use Math.sumPrecise in the scripting implementation

This adds a *very basic* non-MOZCENTRAL polyfill for now, which we should be able to remove once the next QuickJS version is released; note the pending changelog at https://github.com/bellard/quickjs/blob/f1139494d18a2053630c5ed3384a42bb70db3c53/Changelog#L8

Author: Snuffleupagus

src/scripting_api/aform.js

@@ -368,8 +368,8 @@ class AForm {
 
   AFSimple_Calculate(cFunction, cFields) {
     const actions = {
-      AVG: args => args.reduce((acc, value) => acc + value, 0) / args.length,
-      SUM: args => args.reduce((acc, value) => acc + value, 0),
+      AVG: args => Math.sumPrecise(args) / args.length,
+      SUM: args => Math.sumPrecise(args),
       PRD: args => args.reduce((acc, value) => acc * value, 1),
       MIN: args => Math.min(...args),
       MAX: args => Math.max(...args),

src/scripting_api/app_utils.js

@@ -26,6 +26,18 @@ function serializeError(error) {
   return { command: "error", value };
 }
 
+if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
+  // TODO: Remove this once `Math.sumPrecise` is supported in QuickJS.
+  //
+  // Note that this isn't a "proper" polyfill, but since we're only using it to
+  // replace `Array.prototype.reduce()` invocations it should be fine.
+  if (typeof Math.sumPrecise !== "function") {
+    Math.sumPrecise = function (numbers) {
+      return numbers.reduce((a, b) => a + b, 0);
+    };
+  }
+}
+
 export {
   FORMS_VERSION,
   serializeError,