Fix smoke and gravity issue

Smoke used to fly higher with gravity 40 than with gravity 10. Fixed
this by using a start value. At default gravity, light smoke will fly
rise a little higher now, and heavy smoke behaves the same.
With higher gravity the smoke may fall to the ground now.

planet/System.ocg/Particles.c

@@ -67,7 +67,7 @@ global func Particles_Smoke(bool heavy)
 	{
 		CollisionVertex = 500,
 		OnCollision = PC_Stop(),
-		ForceY = PV_Gravity(-100 + heavy*90),
+		ForceY = PV_Gravity(100 + heavy*400, -10),
 		ForceX = PV_Wind(200 - heavy*180),
 		DampingX = 900, DampingY = 900,
 		Alpha = PV_Linear(255, 0),