Function: write_image_file

--Usage--
> write_image_file(some_image, file: filename)

Write an image to a file in the output directory.
If a file with the given name already exists it is overwritten.

Returns the name of the file written.

This function can only be used in an [[type:export template]], when <tt>create directory</tt> is true.

--Parameters--
! Parameter	Type			Description
| @input@	[[type:image]]		Image to write to the file.
| @file@	[[type:string]]		Name of the file to write to
| @width@	[[type:int]]		Width in pixels to use for the image, by default the size of the image is used if available.
| @height@	[[type:int]]		Height in pixels to use for the image, by default the size of the image is used if available.

--Examples--
> write_image_file(file:"image_out.png", linear_blend(...)) == "image_out.png" # image_out.png now contains the given image

--See also--
| [[fun:write_text_file]]	Write a text file to the output directory.
