Fix range-slider bug (index.html Example 2) where hidden but touchable tooltip-max moves up too high.

![tooltip-max-bug](https://cloud.githubusercontent.com/assets/138476/4769027/db36e724-5b73-11e4-9c02-27ccf6f1b284.png)

Above image is bug-reproducing-demo from e4ec897 which I just set `tooltip.opacity=1` forcefully.

In current version, these tooltips are draggable **even when they are invisible**.
I think it is OK for `tooltip-main (center)` and `tooltip-min` to be draggable on hidden because they are very near slider UI, but not torelable for `tootip-MAX`, so I fixed `tootip-MAX`'s top position.