Fix GPUParticle2D offset stutter

(cherry picked from commit 936fd8db3829921ec346461567f235ead4a79582)

scene/resources/particle_process_material.cpp

@@ -761,7 +761,7 @@ void ParticleProcessMaterial::_update_shader() {
 	code += "	float base_angle = (tex_angle) * mix(initial_angle_min, initial_angle_max, angle_rand);\n";
 	code += "	base_angle += CUSTOM.y * LIFETIME * (tex_angular_velocity) * mix(angular_velocity_min,angular_velocity_max, rand_from_seed(alt_seed));\n";
 	code += "	CUSTOM.x = base_angle * degree_to_rad;\n"; // angle
-	code += "	CUSTOM.z = (tex_anim_offset) * mix(anim_offset_min, anim_offset_max, rand_from_seed(alt_seed)) + tv * tex_anim_speed * mix(anim_speed_min, anim_speed_max, rand_from_seed(alt_seed));\n"; // angle
+	code += "	CUSTOM.z = (tex_anim_offset) * mix(anim_offset_min, anim_offset_max, anim_offset_rand) + tv * tex_anim_speed * mix(anim_speed_min, anim_speed_max, rand_from_seed(alt_seed));\n"; // angle
 
 	// apply color
 	// apply hue rotation