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)

2 files changed, 4 insertions, 5 deletions

diff --git a/scene/3d/navigation_region_3d.cpp b/scene/3d/navigation_region_3d.cpp
index b7b1415091..e8cfa05a4b 100644
--- a/scene/3d/navigation_region_3d.cpp
+++ b/scene/3d/navigation_region_3d.cpp
@@ -170,18 +170,17 @@ void _bake_navigation_mesh(void *p_user_data) {
 }
 
 void NavigationRegion3D::bake_navigation_mesh() {
-	ERR_FAIL_COND(bake_thread != nullptr);
+	ERR_FAIL_COND(bake_thread.is_started());
 
 	BakeThreadsArgs *args = memnew(BakeThreadsArgs);
 	args->nav_region = this;
 
-	bake_thread = Thread::create(_bake_navigation_mesh, args);
-	ERR_FAIL_COND(bake_thread == nullptr);
+	bake_thread.start(_bake_navigation_mesh, args);
 }
 
 void NavigationRegion3D::_bake_finished(Ref<NavigationMesh> p_nav_mesh) {
 	set_navigation_mesh(p_nav_mesh);
-	bake_thread = nullptr;
+	bake_thread.wait_to_finish();
 	emit_signal("bake_finished");
 }
 
diff --git a/scene/3d/navigation_region_3d.h b/scene/3d/navigation_region_3d.h
index a5b8c2cd5e..e966523b64 100644
--- a/scene/3d/navigation_region_3d.h
+++ b/scene/3d/navigation_region_3d.h
@@ -46,7 +46,7 @@ class NavigationRegion3D : public Node3D {
 
 	Navigation3D *navigation = nullptr;
 	Node *debug_view = nullptr;
-	Thread *bake_thread = nullptr;
+	Thread bake_thread;
 
 protected:
 	void _notification(int p_what);