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)_.
pdfDocument.getOptionalContentConfig();
      const viewport = page.getViewport({ scale: 1 });
      const canvasAndCtx = CanvasFactory.create(
        viewport.width,
        viewport.height
      );
      const renderTask1 = page.render({
        canvasContext: canvasAndCtx.context,
        canvasFactory: CanvasFactory,
        viewport,
        optionalContentConfigPromise,
      });
      expect(renderTask1 instanceof RenderTask).toEqual(true);
      const renderTask2 = page.render({
        canvasContext: canvasAndCtx.context,
        canvasFactory: CanvasFactory,
        viewport,
        optionalContentConfigPromise,
      });
      expect(renderTask2 instanceof RenderTask).toEqual(true);
      await Promise.all([
        renderTask1.promise,
        renderTask2.promise.then(
          () => {
            
            expect(false).toEqual(true);
          
          reason => {
            // It fails because we are already using this canvas.
            .toEqual(true);