Function: set_mask

--Usage--
> set_mask(image: image, mask: mask_image)

Make parts of an image (more) transparent using a mask.
Black areas in the mask make the image transparent, white areas stay opaque.

The images must have the same size.

--Parameters--
! Parameter	Type			Description
| @image@	[[type:image]]		Image to apply the mask to
| @mask@	[[type:image]]		Mask image.

--Examples--
> set_mask(image: "image1.png", mask: "image3.png")  ==  [[Image]]
>>> set_mask(image: <img src="image1.png" alt='"image1.png"' style="border:1px solid black;vertical-align:middle;margin:1px;" />, mask: <img src="image3.png" alt='"image3.png"' style="border:1px solid black;vertical-align:middle;margin:1px;" />) == <img src="image_set_mask.png" alt='"image_set_mask.png"' style="border:1px solid black;vertical-align:middle;margin:1px;" />

--See also--
| [[fun:set_alpha]]		Change the transparency of an image.
