Target over-penetration for heavy railguns (#3380)

Gadget added that uses a repeatedly spawning projectile to simulate over-penetration of relatively soft targets. Respawned projectiles deal less damage with more gradual loss of impulse. It has some limited customization options for now.

The first unit candidate is the legsrail, which is part of this PR as the unit demo.

luarules/gadgets/unit_collision_damage_behavior.lua

@@ -253,3 +253,23 @@ function gadget:GameFrame(frame)
 	end
 	gameFrame = frame
 end
+
+local function setVelocityControl(unitID, enabled)
+	if enabled == false then
+		launchedUnits[unitID] = nil
+		unitInertiaCheckFlags[unitID] = nil
+	elseif not unitInertiaCheckFlags[unitID] then
+		unitInertiaCheckFlags[unitID] = {
+			expirationFrame = gameFrame + velocityWatchFrames,
+			velocityCap     = unitDefData[Spring.GetUnitDefID(unitID)].velocityCap,
+		}
+	end
+end
+
+function gadget:Initialize()
+	GG.SetVelocityControl = setVelocityControl
+end
+
+function gadget:ShutDown()
+	GG.SetVelocityControl = nil
+end