Fix panning for native Android 4.1 browser.

This particular browser appears to fire a mousemove event as well as the
usual touch events.

The zoom behaviour adds a mousemove listener to reset translate0 to null
(so that scrolling the mouse wheel records a new translate0 vector).
However, the translate0 variable is also used to record initial touch
vectors on touchstart.

The fix uses a different variable to record initial touch vectors, since
in theory it might be possible to fire mousewheel events and touch
events simultaneously.