api: promote the mask() method to Scene from Paint

[canhth]

The mask() function is too general-purpose in thorvg now, making it somewhat over-engineered for practical use. It might not be a bad idea to restrict mask() to a Scene-level scope and optimize the data structure and resources for better efficiency.

Paint::mask() -> Scene::mask()

See: https://github.com/thorvg/thorvg/issues/1372