Enumeration: image combine type

This specifies how an image is to be combined with the background.
This is similair to the feature found in more advanced drawing programs.

The formula's are from [[http://www.pegtop.net/delphi/articles/blendmodes/]].

--Script syntax--
In scripts, combine modes are stored as a string.

--Possible values--
! Value			Description
| @normal@		Normal behaviour, don't combine.
| @add@			 
| @subtract@		 
| @stamp@		 
| @difference@		 
| @negation@		 
| @multiply@		 
| @darken@		 
| @lighten@		 
| @color dodge@		 
| @color burn@		 
| @screen@		 
| @overlay@		 
| @hard light@		 
| @soft light@		 
| @reflect@		 
| @glow@		 
| @freeze@		 
| @heat@		 
| <tt>and</tt>		Bitwise and
| <tt>or</tt>		Bitwise or
| <tt>xor</tt>		Bitwise xot
| @shadow@		Dark colors normally, white colors darken the background.
| @symmetric overlay@	@(overlay(a,b) + overlay(b,a)) / 2@

--Examples--
> combine: overlay
> combine_image(image1: ..., image2: ..., combine: "shadow")

