ShapeRect type

It's somewhat absurd that the only way to do things like gradients on an angle is with a shape, and the only way to do a rectangle in a shape is to specify the rectangle as four separate line elements.

• Ideally, Rectangle should be able to accept a LinearGradient, and the system should do what it needs to do to make that work instead of me having to do it manually with a Shape.
• At a minimum, there should be a ShapeRect {} type to use inside a Shape {} instead of having to manually create a rectangle with ShapePath {}{}.