Fix crash on startup on emscripten

Parts::data was never initialized, and this mattered on emscripten, even though it didn't matter anywhere else.

src/simulation/Simulation.cpp

@@ -976,6 +976,12 @@ int Simulation::parts_avg(int ci, int ni,int t)
 	return PT_NONE;
 }
 
+void Parts::Reset()
+{
+	memset(data.data(), 0, sizeof(Particle)*NPART);
+	lastActiveIndex = 0;
+}
+
 void Simulation::clear_sim(void)
 {
 	for (auto i = 0; i <= parts.lastActiveIndex; i++)
@@ -994,13 +1000,12 @@ void Simulation::clear_sim(void)
 	signs.clear();
 	memset(bmap, 0, sizeof(bmap));
 	memset(emap, 0, sizeof(emap));
-	memset(parts, 0, sizeof(Particle)*NPART);
+	parts.Reset();
 	for (int i = 0; i < NPART-1; i++)
 		parts[i].life = i+1;
 	parts[NPART-1].life = -1;
 	pfree = 0;
 	NUM_PARTS = 0;
-	parts.lastActiveIndex = 0;
 	memset(pmap, 0, sizeof(pmap));
 	memset(fvx, 0, sizeof(fvx));
 	memset(fvy, 0, sizeof(fvy));

src/simulation/Simulation.h

@@ -50,12 +50,21 @@ struct Parts
 		return data.data();
 	}
 
+	Parts()
+	{
+		Reset();
+	}
+
+	Parts(const Parts &other) = default;
+
 	Parts &operator =(const Parts &other)
 	{
 		std::copy(other.data.begin(), other.data.begin() + other.lastActiveIndex + 1, data.begin());
 		lastActiveIndex = other.lastActiveIndex;
 		return *this;
 	}
+
+	void Reset();
 };
 
 struct RenderableSimulation