utils/viewer: fix SVGGraphView.wheelEvent() to work on OSX
utils/viewer: fix SVGGraphView.wheelEvent() to work on OSX

Unit of scrolling on OSX trackpad are way too small so we need to work
with an accumulator.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl