Expose the current animation tree and player to AnimationNode

Exposes the `tree` and `player` members of `AnimationNode::State` available
in C++ to scripts, enabling their use in custom nodes.

Without this various obtuse and potentially problematic workarounds
are required.

For example, with this patch, the following code can be changed from
this (which has undesired side effects):

```gdscript
# seek the animation to its beginning, which will cause `blend_node` to
# return its length
duration = blend_node(animation_node_name, animation_node, 0.0, true, false,
	0.0, AnimationNode.FILTER_IGNORE, true)
```

To the more obvious and direct form:

```gdscript
duration = get_player().get_animation(animation_name).length
```

doc/classes/AnimationNode.xml

@@ -127,6 +127,18 @@
 				Gets the value of a parameter. Parameters are custom local memory used for your nodes, given a resource can be reused in multiple trees.
 			</description>
 		</method>
+		<method name="get_player" qualifiers="const">
+			<return type="AnimationPlayer" />
+			<description>
+				Gets the [AnimationPlayer] referenced by the [AnimationTree] currently processing this node. Returns [code]null[/code] when called outside of [method _process], as multiple trees may reference a single [AnimationNode] resource.
+			</description>
+		</method>
+		<method name="get_tree" qualifiers="const">
+			<return type="AnimationTree" />
+			<description>
+				Gets the [AnimationTree] currently processing this node. Returns [code]null[/code] when called outside of [method _process], as multiple trees may reference a single [AnimationNode] resource.
+			</description>
+		</method>
 		<method name="is_path_filtered" qualifiers="const">
 			<return type="bool" />
 			<argument index="0" name="path" type="NodePath" />

scene/animation/animation_tree.cpp

@@ -403,6 +403,18 @@ Ref<AnimationNode> AnimationNode::get_child_by_name(const StringName &p_name) {
 	return Ref<AnimationNode>();
 }
 
+AnimationTree *AnimationNode::get_tree() const {
+	ERR_FAIL_NULL_V_EDMSG(state, nullptr, "get_tree() called outside of _process()");
+
+	return state->tree;
+}
+
+AnimationPlayer *AnimationNode::get_player() const {
+	ERR_FAIL_NULL_V_EDMSG(state, nullptr, "get_player() called outside of _process()");
+
+	return state->player;
+}
+
 void AnimationNode::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_input_count"), &AnimationNode::get_input_count);
 	ClassDB::bind_method(D_METHOD("get_input_name", "input"), &AnimationNode::get_input_name);
@@ -426,6 +438,9 @@ void AnimationNode::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_parameter", "name", "value"), &AnimationNode::set_parameter);
 	ClassDB::bind_method(D_METHOD("get_parameter", "name"), &AnimationNode::get_parameter);
 
+	ClassDB::bind_method(D_METHOD("get_tree"), &AnimationNode::get_tree);
+	ClassDB::bind_method(D_METHOD("get_player"), &AnimationNode::get_player);
+
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "filter_enabled", PROPERTY_HINT_NONE, "", PROPERTY_USAGE_NO_EDITOR), "set_filter_enabled", "is_filter_enabled");
 	ADD_PROPERTY(PropertyInfo(Variant::ARRAY, "filters", PROPERTY_HINT_NONE, "", PROPERTY_USAGE_NO_EDITOR | PROPERTY_USAGE_INTERNAL), "_set_filters", "_get_filters");
 

scene/animation/animation_tree.h

@@ -154,6 +154,9 @@ class AnimationNode : public Resource {
 
 	virtual Ref<AnimationNode> get_child_by_name(const StringName &p_name);
 
+	AnimationTree *get_tree() const;
+	AnimationPlayer *get_player() const;
+
 	AnimationNode();
 };