use the same task runner for platform&render tasks

Second effort at fixing #133, fixe failed and reported in #326

---
First effort:
This fixes the issue where on MacOS the embedder crashes during window resizing.
fixes #133

This dosn't fixes the jankyness during resize.
Follow flutter/flutter#44136 for more info.

Author: pchampio

application.go

@@ -241,6 +241,8 @@ func (a *Application) Run() error {
 			fmt.Printf("go-flutter: engine.Run() returned result code %d (invalid library version)\n", result)
 		case embedder.ResultInvalidArguments:
 			fmt.Printf("go-flutter: engine.Run() returned result code %d (invalid arguments)\n", result)
+		case embedder.ResultInternalInconsistency:
+			fmt.Printf("go-flutter: engine.Run() returned result code %d (internal inconsistency)\n", result)
 		default:
 			fmt.Printf("go-flutter: engine.Run() returned result code %d (unknown result code)\n", result)
 		}

embedder/embedder.go

@@ -28,6 +28,7 @@ const (
 	ResultSuccess               Result = C.kSuccess
 	ResultInvalidLibraryVersion Result = C.kInvalidLibraryVersion
 	ResultInvalidArguments      Result = C.kInvalidArguments
+	ResultInternalInconsistency Result = C.kInternalInconsistency
 	ResultEngineNotRunning      Result = -1
 )