Make AnimationNodeExtention extend AnimationNode instead of RootNode

doc/classes/AnimationNodeExtension.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<class name="AnimationNodeExtension" inherits="AnimationRootNode" experimental="" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+<class name="AnimationNodeExtension" inherits="AnimationNode" experimental="" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
 	<brief_description>
 		Base class for extending [AnimationRootNode]s from GDScript, C#, or C++.
 	</brief_description>

scene/animation/animation_node_extension.h

@@ -33,8 +33,8 @@
 
 #include "scene/animation/animation_tree.h"
 
-class AnimationNodeExtension : public AnimationRootNode {
-	GDCLASS(AnimationNodeExtension, AnimationRootNode);
+class AnimationNodeExtension : public AnimationNode {
+	GDCLASS(AnimationNodeExtension, AnimationNode);
 
 public:
 	virtual NodeTimeInfo _process(const AnimationMixer::PlaybackInfo p_playback_info, bool p_test_only = false) override;