Implement CollisionPolygon3D margin

(cherry picked from commit fbb1ef759cca66f44523760569f1fcc3f015e2cb)

doc/classes/CollisionPolygon.xml

@@ -17,6 +17,9 @@
 		<member name="disabled" type="bool" setter="set_disabled" getter="is_disabled" default="false">
 			If [code]true[/code], no collision will be produced.
 		</member>
+		<member name="margin" type="float" setter="set_margin" getter="get_margin" default="0.04">
+			The collision margin for the generated [Shape]. See [member Shape.margin] for more details.
+		</member>
 		<member name="polygon" type="PoolVector2Array" setter="set_polygon" getter="get_polygon" default="PoolVector2Array(  )">
 			Array of vertices which define the polygon.
 			[b]Note:[/b] The returned value is a copy of the original. Methods which mutate the size or properties of the return value will not impact the original polygon. To change properties of the polygon, assign it to a temporary variable and make changes before reassigning the [code]polygon[/code] member.

doc/classes/Shape.xml

@@ -13,7 +13,8 @@
 	</methods>
 	<members>
 		<member name="margin" type="float" setter="set_margin" getter="get_margin" default="0.04">
-			The collision margin for the shape.
+			The collision margin for the shape. Used in Bullet Physics only.
+			Collision margins allows collision detection to be more efficient by adding an extra shell around shapes. Collision algorithms are more expensive when objects overlap by more than their margin, so a higher value for margins is better for performance, at the cost of accuracy around edges as it makes them less sharp.
 		</member>
 	</members>
 	<constants>

scene/3d/collision_polygon.cpp

@@ -68,6 +68,7 @@ void CollisionPolygon::_build_polygon() {
 		}
 
 		convex->set_points(cp);
+		convex->set_margin(margin);
 		parent->shape_owner_add_shape(owner_id, convex);
 		parent->shape_owner_set_disabled(owner_id, disabled);
 	}
@@ -162,6 +163,17 @@ bool CollisionPolygon::is_disabled() const {
 	return disabled;
 }
 
+real_t CollisionPolygon::get_margin() const {
+	return margin;
+}
+
+void CollisionPolygon::set_margin(real_t p_margin) {
+	margin = p_margin;
+	if (parent) {
+		_build_polygon();
+	}
+}
+
 String CollisionPolygon::get_configuration_warning() const {
 
 	String warning = Spatial::get_configuration_warning();
@@ -196,11 +208,15 @@ void CollisionPolygon::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_disabled", "disabled"), &CollisionPolygon::set_disabled);
 	ClassDB::bind_method(D_METHOD("is_disabled"), &CollisionPolygon::is_disabled);
 
+	ClassDB::bind_method(D_METHOD("set_margin", "margin"), &CollisionPolygon::set_margin);
+	ClassDB::bind_method(D_METHOD("get_margin"), &CollisionPolygon::get_margin);
+
 	ClassDB::bind_method(D_METHOD("_is_editable_3d_polygon"), &CollisionPolygon::_is_editable_3d_polygon);
 
 	ADD_PROPERTY(PropertyInfo(Variant::REAL, "depth"), "set_depth", "get_depth");
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "disabled"), "set_disabled", "is_disabled");
 	ADD_PROPERTY(PropertyInfo(Variant::POOL_VECTOR2_ARRAY, "polygon"), "set_polygon", "get_polygon");
+	ADD_PROPERTY(PropertyInfo(Variant::REAL, "margin", PROPERTY_HINT_RANGE, "0.001,10,0.001"), "set_margin", "get_margin");
 }
 
 CollisionPolygon::CollisionPolygon() {

scene/3d/collision_polygon.h

@@ -38,6 +38,7 @@ class CollisionObject;
 class CollisionPolygon : public Spatial {
 
 	GDCLASS(CollisionPolygon, Spatial);
+	real_t margin = 0.04;
 
 protected:
 	float depth;
@@ -71,6 +72,9 @@ public:
 
 	virtual AABB get_item_rect() const;
 
+	real_t get_margin() const;
+	void set_margin(real_t p_margin);
+
 	String get_configuration_warning() const;
 
 	CollisionPolygon();