fixes for Android transparency and add removeAsset method

android/src/main/cpp/filament_android.cpp

@@ -169,5 +169,9 @@ extern "C" {
   void release_source_assets(void* viewer) {
     ((FilamentViewer*)viewer)->releaseSourceAssets();
   }
+
+  void remove_asset(void* viewer) {
+    ((FilamentViewer*)viewer)->removeAsset();
+  }
   
 }

android/src/main/kotlin/app/polyvox/filament/FilamentInterop.kt

@@ -62,4 +62,7 @@ interface FilamentInterop : Library {
 
     fun release_source_assets(viewer:Pointer);
 
+    fun remove_asset(viewer:Pointer);
+
+
 }

android/src/main/kotlin/app/polyvox/filament/FilamentView.kt

@@ -104,8 +104,8 @@ PlatformView  {
         _methodChannel.invokeMethod("ready", null)
 
         choreographer = Choreographer.getInstance()
-            
-        _view.setZOrderOnTop(true)
+        
+        _view.setZOrderOnTop(false)            
         _view.holder.setFormat(PixelFormat.OPAQUE)
   
         _view.holder.addCallback (object : SurfaceHolder.Callback {
@@ -301,6 +301,10 @@ PlatformView  {
               _lib.release_source_assets(_viewer!!)
               result.success("OK");
             } 
+            "removeAsset" -> {
+              _lib.remove_asset(_viewer!!)
+              result.success("OK");
+            } 
             "playAnimation" -> {
               val args = call.arguments as ArrayList<Any?>
               _lib.play_animation(_viewer!!, args[0] as Int, args[1] as Boolean)