Added the SvgMask-related implementations (v4.9.0)

[simon-ritchie]

Since version 4.9.0, the SvgMask-related implementations, such as the SvgMask class, svg_mask property, and add_svg_masking_object method, have been added.

You can set another SVG DisplayObject as a mask for an SVG DisplayObject (e.g., Rectangle) to display only the overlapping area.

For more details, please see the following document:

• SvgMask class and its related interfaces