Add primitive 2D meshes

Allows drawing (anti-aliased) 2D shapes using MeshInstance2D.
Based on the code for primitive 3D meshes.

doc/classes/CapsuleMesh2D.xml

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+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="CapsuleMesh2D" inherits="PrimitiveMesh2D" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+		A capsule-shaped [PrimitiveMesh2D].
+	</brief_description>
+	<description>
+		A capsule-shaped [PrimitiveMesh2D].
+	</description>
+	<tutorials>
+	</tutorials>
+	<members>
+		<member name="height" type="float" setter="set_height" getter="get_height" default="30.0">
+			Total height of the capsule mesh (including the semicircular ends).
+		</member>
+		<member name="radial_segments" type="int" setter="set_radial_segments" getter="get_radial_segments" default="32">
+			Number of radial segments on each semicircle of the capsule mesh.
+		</member>
+		<member name="radius" type="float" setter="set_radius" getter="get_radius" default="10.0">
+			Radius of the semicircles on the ends of the capsule mesh.
+		</member>
+	</members>
+</class>

doc/classes/CircleMesh2D.xml

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+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="CircleMesh2D" inherits="PrimitiveMesh2D" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+		A circular [PrimitiveMesh2D].
+	</brief_description>
+	<description>
+		A circular [PrimitiveMesh2D].
+	</description>
+	<tutorials>
+	</tutorials>
+	<members>
+		<member name="radial_segments" type="int" setter="set_radial_segments" getter="get_radial_segments" default="64">
+			Number of radial segments on the circle.
+		</member>
+		<member name="radius" type="float" setter="set_radius" getter="get_radius" default="10.0">
+			Radius of the circle.
+		</member>
+	</members>
+</class>

doc/classes/PrimitiveMesh2D.xml

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+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="PrimitiveMesh2D" inherits="Mesh" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+		Base class for all primitive 2D meshes.
+	</brief_description>
+	<description>
+		Base class for all primitive 2D meshes. Examples include [RectangleMesh2D], [CircleMesh2D], and [CapsuleMesh2D].
+	</description>
+	<tutorials>
+	</tutorials>
+	<methods>
+		<method name="_create_mesh_array" qualifiers="virtual const">
+			<return type="Array" />
+			<description>
+				Override this method to customize how this primitive mesh should be generated. Should return an [Array] where each element is another Array of values required for the mesh (see the [enum Mesh.ArrayType] constants).
+			</description>
+		</method>
+		<method name="get_mesh_arrays" qualifiers="const">
+			<return type="Array" />
+			<description>
+				Returns the mesh arrays used to make up the surface of this primitive mesh.
+			</description>
+		</method>
+		<method name="request_update">
+			<return type="void" />
+			<description>
+				Request an update of this primitive mesh based on its properties.
+			</description>
+		</method>
+	</methods>
+	<members>
+		<member name="antialiased" type="bool" setter="set_antialiased" getter="is_antialiased" default="false">
+			If [code]true[/code], a half transparent "skirt" will be attached to the boundary, making outlines smooth.
+		</member>
+		<member name="custom_aabb" type="AABB" setter="set_custom_aabb" getter="get_custom_aabb" default="AABB(0, 0, 0, 0, 0, 0)">
+			Overrides the [AABB] with one defined by user for use with frustum culling. Especially useful to avoid unexpected culling when using a shader to offset vertices.
+		</member>
+	</members>
+</class>

doc/classes/RectangleMesh2D.xml

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+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="RectangleMesh2D" inherits="PrimitiveMesh2D" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+		A rectangular [PrimitiveMesh2D].
+	</brief_description>
+	<description>
+		A rectangular [PrimitiveMesh2D].
+	</description>
+	<tutorials>
+	</tutorials>
+	<members>
+		<member name="size" type="Vector2" setter="set_size" getter="get_size" default="Vector2(20, 20)">
+			The rectangle's width and height.
+		</member>
+		<member name="subdivide_height" type="int" setter="set_subdivide_height" getter="get_subdivide_height" default="0">
+			Number of extra edge loops inserted along the Y axis.
+		</member>
+		<member name="subdivide_width" type="int" setter="set_subdivide_width" getter="get_subdivide_width" default="0">
+			Number of extra edge loops inserted along the X axis.
+		</member>
+	</members>
+</class>