Make 'size_override_stretch' a property in 'Viewport'

doc/classes/Viewport.xml

@@ -284,6 +284,9 @@
 		<member name="size" type="Vector2" setter="set_size" getter="get_size" default="Vector2( 0, 0 )">
 			The width and height of viewport.
 		</member>
+		<member name="size_override_stretch" type="bool" setter="set_size_override_stretch" getter="is_size_override_stretch_enabled" default="false">
+			If [code]true[/code], the size override affects stretch as well.
+		</member>
 		<member name="transparent_bg" type="bool" setter="set_transparent_background" getter="has_transparent_background" default="false">
 			If [code]true[/code], the viewport should render its background as transparent.
 		</member>

scene/main/viewport.cpp

@@ -3064,6 +3064,7 @@ void Viewport::_bind_methods() {
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "arvr"), "set_use_arvr", "use_arvr");
 
 	ADD_PROPERTY(PropertyInfo(Variant::VECTOR2, "size"), "set_size", "get_size");
+	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "size_override_stretch"), "set_size_override_stretch", "is_size_override_stretch_enabled");
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "own_world"), "set_use_own_world", "is_using_own_world");
 	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "world", PROPERTY_HINT_RESOURCE_TYPE, "World"), "set_world", "get_world");
 	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "world_2d", PROPERTY_HINT_RESOURCE_TYPE, "World2D", 0), "set_world_2d", "get_world_2d");