Fixed BetterCharacterControl "flickering" when not moving.

Sometimes, when the BetterCharacterControl is not moving, it flickers due it physics being always active. The reason for it being always active is that on the prePhysicsTick method it always set the rigidBody's linear velocity (and the method PhysicsRigidBody.setLinearVelocity(Vector3f) reactivates the physics each time is called).

The fix consist on just comparing if the current velocity and the setting one aren't the same and, thus, not setting it (using ZERO_TOLERANCE instead 0 to best results).

jme3-bullet/src/common/java/com/jme3/bullet/control/BetterCharacterControl.java

@@ -171,6 +171,8 @@ public class BetterCharacterControl extends AbstractPhysicsControl implements Ph
         }
         TempVars vars = TempVars.get();
 
+        Vector3f currentVelocity = vars.vect2.set(velocity);
+        
         // dampen existing x/z forces
         float existingLeftVelocity = velocity.dot(localLeft);
         float existingForwardVelocity = velocity.dot(localForward);
@@ -194,7 +196,7 @@ public class BetterCharacterControl extends AbstractPhysicsControl implements Ph
             //add resulting vector to existing velocity
             velocity.addLocal(localWalkDirection);
         }
-        rigidBody.setLinearVelocity(velocity);
+        if(currentVelocity.distance(velocity) > FastMath.ZERO_TOLERANCE) rigidBody.setLinearVelocity(velocity);
         if (jump) {
             //TODO: precalculate jump force
             Vector3f rotatedJumpForce = vars.vect1;