TPT: Prevent THDR from counting towards stacked particle limit

src/simulation/Simulation.cpp

@@ -4156,6 +4156,7 @@ void Simulation::update_particles()//doesn't update the particles themselves, bu
 					// To make particles collide correctly when inside these elements, these elements must not overwrite an existing pmap entry from particles inside them
 					if (!pmap[y][x] || (t!=PT_INVIS && t!= PT_FILT))
 						pmap[y][x] = t|(i<<8);
+					if (t!=PT_THDR)
 						pmap_count[y][x]++;
 				}
 			}