Scrolling with trackpad multi-touch should behave more like in OSX

[Matthew Barnard]

In OSX, the multi-touch trackpad (e.g. on Macbooks) can be used to scroll by holding down the trackpad button with, say, the thumb and scrolling with another finger. This works in Ubuntu, but minor variations in the positioning of the thumb affect the delta of the scroll. In OSX, once this action is engaged the motion of the stable touch (the thumb) is mostly ignored. This has the effect of making scrolling much more accurate and smooth because the motion is entirely dependent on the moving finger. In Ubuntu, scrolling like this is generally wonky if the thumb moves at all on the trackpad.