SmokeSource: Allow smoke particles to increase in vertical speed over time

[MeteorSwarm]

Currently, SmokeSource only allows smoke particles to start at a given vertical speed and slow down over time. The opposite, starting at a given speed and increasing over time, is not currently possible, which makes certain effects like bubbles underwater impossible to realistically render, as Maraxsis attempts to do with the existing API.

To address this request, allow vertical speed of smoke sources to increase over time, either by allowing negative values for vertical_speed_slowdown or by adding another field.