Fix some particles flickering when SRT is enabled

Broken since c645269c86, where we started giving the renderer thread only the live part of Simulation::parts. Or at least intended to, but ended up copying whatever amount we should have copied the previous time around. The flickering was hard to notice because the number of particles rarely changes much between frames.

src/simulation/Simulation.h

@@ -52,7 +52,7 @@ struct Parts
 
 	Parts &operator =(const Parts &other)
 	{
-		std::copy(other.data.begin(), other.data.begin() + lastActiveIndex + 1, data.begin());
+		std::copy(other.data.begin(), other.data.begin() + other.lastActiveIndex + 1, data.begin());
 		lastActiveIndex = other.lastActiveIndex;
 		return *this;
 	}