trackpad scrolling not proportional to gesture velocity

JAL-2930 addressed a serious defect with trackpad scrolling that made it unusable on java 1.8+152 onwards, and 1.9. The fix was simple - but because of the change in trackpad eventing to use the 'precise scroll' count, jalview nudges the alignment precisely one row or column regardless of the value of the scroll delta.
In previous JREs, low precision integer scroll mousewheel events were simulated on OSX (at least) to implement momentum swipe effects (ie scrolling repeatedly in one direction would make it scroll faster, and gradually slow down), but the JAL-2930 patch which switches to precise scroll values has a counterintuitive behaviour: swiping very slowly across trackpad in a direction will scroll the alignment *farther* than swiping quickly (because more events arise from a longer swipe than a shorter one).