Update Grab to pan to fix performance issue

Reported at http://stackoverflow.com/questions/21392844/pdf-js-recalculating-problme
See also https://github.com/Rob--W/grab-to-pan.js/commit/e0f893753ac10a5c97159fd01155b05d234f4c0f


To verify that this patch improves performance, just follow the following steps:

1. Build the latest version of PDF.js and open the PDF Viewer.
2. Scroll through the whole document (tracemonkey) to get all pages rendered.
3. Activate the hand tool (press <kbd>H</kbd>).
3. Open the developer tools (Chrome).
4. Go to the Timeline tab and start the timeline.
5. Click inside the PDF and move the mouse a little bit.
6. Look at the purple bar ("recalculating styles") to see how much time it took to change the cursor ("Recalculate Style (viewer.js:2201)" (with a Call Site Stack to GrabToPan__onmousedown/move)).

Before this patch, my laptop reported ~35 ms, after the fix, the time went down to 0.3ms. For large documents, the performance improvement is even more significant.