[xna] Fixed multi pass rending (was not working, did nothing after first pass). Closes #1554.

spine-xna/src/MeshBatcher.cs

@@ -162,6 +162,9 @@ namespace Spine {
 				freeItems.Enqueue(item);
 			}
 			FlushVertexArray(device, vertexCount, triangleCount);
+		}
+
+		public void AfterLastDrawPass () {
 			items.Clear();
 		}
 

spine-xna/src/SkeletonRenderer.cs

@@ -86,6 +86,7 @@ namespace Spine {
 				pass.Apply();
 				batcher.Draw(device);
 			}
+			batcher.AfterLastDrawPass();
 		}
 
 		public void Draw(Skeleton skeleton) {