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

Changes versus grab-to-pan.js: Moved inline style to viewer.css,
lowered z-index from 2e9 to 50000 (note to future maintainers:
make sure that all other z-index values stay below 50000!).
Merge pull request #4209 from Rob--W/grab-to-pan-performance

Update Grab to pan to fix performance issue