Linux: Add fl_engine_poll_task_runner. (#7) (#8)

Exposes polling functionality from Linux task runner. Required to
implement resize synchronization.

Author: knopp

engine/src/flutter/shell/platform/linux/fl_engine.cc

@@ -583,3 +583,9 @@ void fl_engine_execute_task(FlEngine* self, FlutterTask* task) {
     g_warning("Failed to run task");
   }
 }
+
+G_MODULE_EXPORT void fl_engine_poll_task_runner(FlEngine* self) {
+  g_return_if_fail(FL_IS_ENGINE(self));
+
+  fl_task_runner_wait(self->task_runner);
+}

engine/src/flutter/shell/platform/linux/public/flutter_linux/fl_engine.h

@@ -70,6 +70,13 @@ gboolean fl_engine_start(FlEngine* engine, GError** error);
  */
 FlBinaryMessenger* fl_engine_get_binary_messenger(FlEngine* engine);
 
+/*
+ * Blocks until a Flutter message is posted on task runner and then processes
+ * it. Must be called on platform thread. During polling no other system
+ * messages are processed.
+ */
+void fl_engine_poll_task_runner(FlEngine* self);
+
 G_END_DECLS
 
 #endif  // FLUTTER_SHELL_PLATFORM_LINUX_PUBLIC_FLUTTER_LINUX_FL_ENGINE_H_