Function: set_metadata

--Usage--
> set_metadata(input: image, metadata: string)

Returns a copy of the input image with the metadata set to the given string.

Metadata is only stored when an image is saved as a PNG.
It is saved in a uncompressed iTXt chunk with the key "Description".
If the image is saved as a JPG, the metadata is lost.

--Parameters--
! Parameter	Type				Description
| @input@	[[type:image]]		The image that will be copied.
| @metadata@	[[type:string]]		The metadata that will be stored in the copy.
| @set@		[[type:set]]		The active set. This can be omitted since 'set' is a predefined variable.