Particle system improvements:

- particles can collide with the outer walls of the sub
- drag + waterdrag parameters
- fixed particles "twitching" when the submarine is moving fast
- submarine velocity only affects the initial velocity of the particles (i.e. accelerating the sub doesn't automatically accelerate them)

Subsurface/Source/Map/Gap.cs

@@ -163,6 +163,8 @@ namespace Barotrauma
                 hulls[1] = temp;
             }
 
+            flowTargetHull = hulls[0];
+
             for (int i = 0 ; i <2; i++)
             {
                 if (hulls[i]==null) continue;