Add origin parameter to updateScale

This parameter allows defining which point should remain
fixed while scaling the document. It can be used, for example,
to implement "zoom around the cursor" or "zoom around
pinch center".

The logic was previously implemented in `web/app.js`, but
moving it to the viewer scaling utilities themselves makes it
easier to implement similar zooming functionalities in
other embedders.

test/integration/viewer_spec.mjs

@@ -17,10 +17,90 @@ import {
   awaitPromise,
   closePages,
   createPromise,
+  getSpanRectFromText,
   loadAndWait,
 } from "./test_utils.mjs";
 
 describe("PDF viewer", () => {
+  describe("Zoom origin", () => {
+    let pages;
+
+    beforeAll(async () => {
+      pages = await loadAndWait(
+        "tracemonkey.pdf",
+        ".textLayer .endOfContent",
+        "page-width",
+        null,
+        { page: 2 }
+      );
+    });
+
+    afterAll(async () => {
+      await closePages(pages);
+    });
+
+    async function getTextAt(page, pageNumber, coordX, coordY) {
+      await page.waitForFunction(
+        pageNum =>
+          !document.querySelector(
+            `.page[data-page-number="${pageNum}"] > .textLayer`
+          ).hidden,
+        {},
+        pageNumber
+      );
+      return page.evaluate(
+        (x, y) => document.elementFromPoint(x, y)?.textContent,
+        coordX,
+        coordY
+      );
+    }
+
+    it("supports specifiying a custom origin", async () => {
+      await Promise.all(
+        pages.map(async ([browserName, page]) => {
+          // We use this text span of page 2 because:
+          // - it's in the visible area even when zooming at page-width
+          // - it's small, so it easily catches if the page moves too much
+          // - it's in a "random" position: not near the center of the
+          //   viewport, and not near the borders
+          const text = "guards";
+
+          const rect = await getSpanRectFromText(page, 2, text);
+          const originX = rect.x + rect.width / 2;
+          const originY = rect.y + rect.height / 2;
+
+          await page.evaluate(
+            origin => {
+              window.PDFViewerApplication.pdfViewer.increaseScale({
+                scaleFactor: 2,
+                origin,
+              });
+            },
+            [originX, originY]
+          );
+          const textAfterZoomIn = await getTextAt(page, 2, originX, originY);
+          expect(textAfterZoomIn)
+            .withContext(`In ${browserName}, zoom in`)
+            .toBe(text);
+
+          await page.evaluate(
+            origin => {
+              window.PDFViewerApplication.pdfViewer.decreaseScale({
+                scaleFactor: 0.8,
+                origin,
+              });
+            },
+            [originX, originY]
+          );
+          const textAfterZoomOut = await getTextAt(page, 2, originX, originY);
+          expect(textAfterZoomOut)
+            .withContext(`In ${browserName}, zoom out`)
+            .toBe(text);
+        })
+      );
+    });
+  });
+
   describe("Zoom with the mouse wheel", () => {
     let pages;