Add QuadMesh back as a subclass of PlaneMesh.

This simplifies the creation of billboarded meshes without any code overhead.

doc/classes/PlaneMesh.xml

@@ -31,10 +31,10 @@
 			[PlaneMesh] will face the positive X-axis.
 		</constant>
 		<constant name="FACE_Y" value="1" enum="Orientation">
-			[PlaneMesh] will face the positive Y-axis. This matches the behaviour of the [PlaneMesh] in Godot 3.x.
+			[PlaneMesh] will face the positive Y-axis. This matches the behavior of the [PlaneMesh] in Godot 3.x.
 		</constant>
 		<constant name="FACE_Z" value="2" enum="Orientation">
-			[PlaneMesh] will face the positive Z-axis. This matches the behvaiour of the QuadMesh in Godot 3.x.
+			[PlaneMesh] will face the positive Z-axis. This matches the behavior of the QuadMesh in Godot 3.x.
 		</constant>
 	</constants>
 </class>

doc/classes/QuadMesh.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="QuadMesh" inherits="PlaneMesh" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+		Class representing a square mesh facing the camera.
+	</brief_description>
+	<description>
+		Class representing a square [PrimitiveMesh]. This flat mesh does not have a thickness. By default, this mesh is aligned on the X and Y axes; this rotation is more suited for use with billboarded materials. A [QuadMesh] is equivalent to a [PlaneMesh] except its default [member PlaneMesh.orientation] is [constant PlaneMesh.FACE_Z].
+	</description>
+	<tutorials>
+		<link title="GUI in 3D Demo">https://godotengine.org/asset-library/asset/127</link>
+		<link title="2D in 3D Demo">https://godotengine.org/asset-library/asset/129</link>
+	</tutorials>
+	<members>
+		<member name="orientation" type="int" setter="set_orientation" getter="get_orientation" overrides="PlaneMesh" enum="PlaneMesh.Orientation" default="2" />
+	</members>
+</class>

scene/register_scene_types.cpp

@@ -794,6 +794,7 @@ void register_scene_types() {
 	GDREGISTER_CLASS(CylinderMesh);
 	GDREGISTER_CLASS(PlaneMesh);
 	GDREGISTER_CLASS(PrismMesh);
+	GDREGISTER_CLASS(QuadMesh);
 	GDREGISTER_CLASS(SphereMesh);
 	GDREGISTER_CLASS(TextMesh);
 	GDREGISTER_CLASS(TorusMesh);
@@ -959,7 +960,6 @@ void register_scene_types() {
 	ClassDB::add_compatibility_class("Navigation3D", "Node3D");
 	ClassDB::add_compatibility_class("Navigation2D", "Node2D");
 	ClassDB::add_compatibility_class("OpenSimplexNoise", "FastNoiseLite");
-	ClassDB::add_compatibility_class("QuadMesh", "PlaneMesh");
 	ClassDB::add_compatibility_class("ToolButton", "Button");
 	ClassDB::add_compatibility_class("YSort", "Node2D");
 	// Portal and room occlusion was replaced by raster occlusion (OccluderInstance3D node).

scene/resources/primitive_meshes.h

@@ -262,6 +262,18 @@ public:
 
 VARIANT_ENUM_CAST(PlaneMesh::Orientation)
 
+/*
+	A flat rectangle, inherits from PlaneMesh but defaults to facing the Z-plane.
+*/
+class QuadMesh : public PlaneMesh {
+	GDCLASS(QuadMesh, PlaneMesh);
+
+public:
+	QuadMesh() {
+		set_orientation(FACE_Z);
+	}
+};
+
 /**
 	A prism shapen, handy for ramps, triangles, etc.
 */