Fix memory leak on iOS (#789)

* Fix memory leak on iOS

* Add check for caDisplayLink being disposed before drawing

* Add logger warn

skiko/src/iosMain/kotlin/org/jetbrains/skiko/SkiaLayer.ios.kt

@@ -50,6 +50,10 @@ actual open class SkiaLayer {
 
     actual fun detach() {
         view?.detach()
+
+        // GC bug? fixes leak on iOS
+        view = null
+        skikoView = null
     }
 
     actual var skikoView: SkikoView? = null

skiko/src/iosMain/kotlin/org/jetbrains/skiko/redrawer/MetalRedrawer.ios.kt

@@ -101,6 +101,11 @@ internal class MetalRedrawer(
     }
 
     private fun draw() {
+        if (caDisplayLink == null) {
+            Logger.warn { "caDisplayLink callback called after it was invalidated "}
+            return
+        }
+
         autoreleasepool {
             val (width, height) = metalLayer.drawableSize.useContents {
                 width.roundToInt() to height.roundToInt()