Introduce a new type MetaCursorSpriteXcursor that is a MetaCursorSprite
implementation backed by Xcursor images. A plain MetaCursorSprite can
still be created "bare bone", but must be manually provided with a
texture. These usages will eventually be wrapped into new
MetaCursorSprite types while turning MetaCursorSprite into an abstract
type.
https://gitlab.gnome.org/GNOME/mutter/issues/77
