Adds a new Frame to this Texture.
A Frame is a rectangular region of a TextureSource with a unique index or string-based key.
The name given must be unique within this Texture. If it already exists, this method will return
|name||number | string||
The name of this Frame. The name is unique within the Texture.
The index of the TextureSource that this Frame is a part of.
The x coordinate of the top-left of this Frame.
The y coordinate of the top-left of this Frame.
The width of this Frame.
The height of this Frame.
The Frame that was added to this Texture, or
null if the given name already exists.