Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

modules/opensimplex/noise_texture.h

@@ -45,7 +45,7 @@ class NoiseTexture : public Texture2D {
 private:
 	Ref<Image> data;
 
-	Thread *noise_thread;
+	Thread noise_thread;
 
 	bool first_time;
 	bool update_queued;