fixed a bug with dangling pointer on loadscreen

src/net/torvald/terrarum/gameworld/BlockLayer.kt

@@ -23,6 +23,9 @@ open class BlockLayer(val width: Int, val height: Int) : Disposable {
     // using unsafe pointer gets you 100 fps, whereas using directbytebuffer gets you 90
     internal val ptr: UnsafePtr = UnsafeHelper.allocate(width * height * BYTES_PER_BLOCK)
 
+    val ptrDestroyed: Boolean
+        get() = ptr.destroyed
+
     init {
         ptr.fillWith(0)
     }