Fix some synchronization flaws

- shader: add barrier at the end of each iteration to avoid writing before the reading finished
- cpp: add synchronization between compute and graphics, the image barrier is not enough for the data visibility between different submission

	modified:   data/shaders/computenbody/particle_calculate.comp
	modified:   data/shaders/computenbody/particle_calculate.comp.spv
	modified:   examples/computenbody/computenbody.cpp

data/shaders/computenbody/particle_calculate.comp

@@ -62,6 +62,9 @@ void main()
 			vec3 len = other.xyz - position.xyz;
 			acceleration.xyz += GRAVITY * len * other.w / pow(dot(len, len) + SOFTEN, POWER);
 		}
+
+		memoryBarrierShared();
+		barrier();
 	}
 
 	particles[index].vel.xyz += ubo.deltaT * acceleration.xyz;