An official mask object, or behaviour which could be applied to a sprite object to make it into a mask.
The masked object would render as transparent (in the shape of the sprite image) and any other object on the same layer overlapping it with a lower z order number would be hidden if behind it, or partially obscured by it when partially overlapping, while any objects on layers below it would still be rendered. i.e objects would be hidden behind an invisible object.
Currently to achieve this effect requires the use of blend modes and force own texture.
This should already be possible using blend modes.