Add a worker thread to support async tasks from the player thread

(and maybe others later)

src/worker.h

@@ -0,0 +1,28 @@
+
+#ifndef __WORKER_H__
+#define __WORKER_H__
+
+/* The worker thread is made for running asyncronous tasks from a real time
+ * thread, mainly the player thread.
+
+ * The worker_execute() function will trigger a callback from the worker thread.
+ * Before returning the function will copy the argument given, so the caller
+ * does not need to preserve them. However, if the argument contains pointers to
+ * data, the caller must either make sure that the data remains valid until the
+ * callback (which can free it), or make sure the callback does not refer to it.
+ *
+ * @param cb the function to call from the worker thread
+ * @param cb_arg arguments for callback
+ * @param arg_size size of the arguments given
+ * @param delay how much in seconds to delay the execution
+ */
+void
+worker_execute(void (*cb)(void *), void *cb_arg, size_t arg_size, int delay);
+
+int
+worker_init(void);
+
+void
+worker_deinit(void);
+
+#endif /* !__WORKER_H__ */