For OS X 10.7+, use scrollingDeltaY instead of deltaY.
The scrollingDeltaY parameter provides more precise scrolling on enabled devices. However, since VTK's MouseWheel events do not include a delta value, we must accumulate the precise delta and emit an event whenever the accumulated delta exceeds a specific threshold. This provides conversion from Cocoa's precise scroll increments to VTK's coarse scroll increments.