Fixed a server-side off-by-one error in the player input IDs

I'm guessing this one comes from the player's position not being updated immediately after the input is processed, so that's where most of the remaining error came from.
Also added some rounding to the horizontal velocity when approaching 0, and changed something that depended on the head limb to use the collider instead, which should hopefully further reduce the chance of syncing errors.

Subsurface/Source/Characters/Animation/HumanoidAnimController.cs

@@ -489,6 +489,10 @@ namespace Barotrauma
             {
                 movement = Vector2.Normalize(movement);
             }
+            if (Math.Abs(movement.X)<0.005f)
+            {
+                movement.X = 0.0f;
+            }
         }
 
         private void ClimbOverObstacles()