Function: set_alpha

--Usage--
> set_alpha(input: image, alpha: transparency value)

Make an image more transparent.

The transprarency is multiplied by @alpha@.
If @alpha@ is 1 the image stays the same, if @alpha@ is 0 the image becomes completely transprent.

--Parameters--
! Parameter	Type			Description
| @input@	[[type:image]]		Image to make transparent.
| @alpha@	[[type:double]]		Transparency factor.

--Examples--
> set_alpha("image1.png", alpha: 0.5)  ==  [[Image]]
>>> set_mask(<img src="image1.png" alt='"image1.png"' style="border:1px solid black;vertical-align:middle;margin:1px;" />, alpha: 0.5) == <img src="image_set_alpha.png" alt='"image_set_alpha.png"' style="border:1px solid black;vertical-align:middle;margin:1px;" />

--See also--
| [[fun:set_mask]]		Set the transparancy mask of an image.
