Fix custom w component being uninitialized on CPU particles.

(cherry picked from commit 38085f2f6982c491935a434bb45e358dbebe1714)
(cherry picked from commit b9c280b73ff6a13ea490d2da0f2728bcef3038dc)
(cherry picked from commit 895ed2aed7baf1a6e32da39f7e2561973f500344)

scene/3d/cpu_particles.cpp

@@ -69,6 +69,7 @@ void CPUParticles::set_amount(int p_amount) {
 
 		for (int i = 0; i < p_amount; i++) {
 			w[i].active = false;
+			w[i].custom[3] = 0.0; // Make sure w component isn't garbage data
 		}
 	}