add 'fill_image' script function, rename 'offset' combine mode to 'lightmap'

doc/function/fill_image.txt

@@ -0,0 +1,14 @@
+Function: fill_image
+
+DOC_MSE_VERSION: since 2.6.0
+
+--Usage--
+> fill_image(input: image, color: color)
+
+Fill an image with a single color, but keep the alpha channel unchanged.
+
+
+--Parameters--
+! Parameter	Type			Description
+| @input@	[[type:image]]		Image to fill.
+| @color@	[[type:color]]		Color to fill with.

doc/function/index.txt

@@ -86,6 +86,7 @@ These functions are built into the program, other [[type:function]]s can be defi
 | [[fun:set_mask]]		Set the transparancy mask of an image.
 | [[fun:set_alpha]]		Change the transparency of an image.
 | [[fun:set_combine]]		Change how the image should be combined with the background.
+| [[fun:fill_image]]		Fill an image with a single color.
 | [[fun:saturate_image]]		Saturate/desaturate an image.
 | [[fun:brighten_image]]		Brighten/darken an image.
 | [[fun:invert_image]]		Invert the colors of an image.