Hello!

Let me know if i should have opened this issue on the [pdfjs-dist](https://github.com/mozilla/pdfjs-dist) repo but I though here would fit.

I'm using pdfjs-dist and I'm in need of some more types to be exported. I have no idea how the types end up in the dist-repo but would love to open a PR if only someone could point me in the right direction 😃 Since I was using an older version of pdfjs-dist I realize the only type I need that's not in the type defs in the latest release are _PageViewport_ from _[display/display_utils](https://github.com/mozilla/pdf.js/blob/master/src/display/display_utils.js#L153)_.
it("caches image resources at the document/page level as expected (issue 11878)", async function () {
      const { NUM_PAGES_THRESHOLD } = GlobalImageCache,
        EXPECTED_WIDTH = 2550,
        EXPECTED_HEIGHT = 3300;
      const loadingTask = getDocument(buildGetDocumentParams("issue11878.pdf"));
      const pdfDoc = await loadingTask.promise;
      let firstImgData = null;
      for (let i = 1; i <= pdfDoc.numPages; i++) {
        const pdfPage = await pdfDoc.getPage(i);
        const opList = await pdfPage.getOperatorList();
        const { commonObjs, objs } = pdfPage;
        const imgIndex = opList.fnArray.indexOf(OPS.paintImageXObject);
        const [objId, width, height] = opList.argsArray[imgIndex];
        if (i < NUM_PAGES_THRESHOLD) {
          expect(objId).toEqual(`img_p${i - 1}_1`);
          expect(objs.has(objId)).toEqual(true);
          expect(commonObjs.has(objId)).toEqual(false);
        } else {
          expect(objId).toEqual(
            `g_${loadingTask.docId}_img_p${NUM_PAGES_THRESHOLD - 1}_1`
          );
          expect(objs.has(objId)).toEqual(false);
          expect(commonObjs.has(objId)).toEqual(true);
        }
        expect(width).toEqual(EXPECTED_WIDTH);
        expect(height).toEqual(EXPECTED_HEIGHT);
        // Ensure that the actual image data is identical for all pages.
        if (i === 1) {
          firstImgData = objs.get(objId);
          expect(firstImgData.width).toEqual(EXPECTED_WIDTH);