Adds a Sprite Sheet to this Texture Manager, where the Sprite Sheet exists as a Frame within a Texture Atlas.
In Phaser terminology a Sprite Sheet is a texture containing different frames, but each frame is the exact same size and cannot be trimmed or rotated.
The unique string-based key of the Texture.
The configuration object for this Sprite Sheet.
The Texture that was created, or
null if the key is already in use.