Add new function "dither".

* scripts/image/dither.m: Add new function.
* scripts/image/module.mk: Add new function to build system.
* scripts/help/__unimplemented__.m: Remove function from list of unimplemented
functions.
* doc/interpreter/image.txi: Add documention of new function to manual.
* etc/NEWS.11: Add name to list of new functions.

See: #33

Author: leolca

doc/interpreter/image.txi

@@ -153,6 +153,10 @@ formats.
 
 @DOCSTRING(ind2rgb)
 
+The following function allows processing an existing image.
+
+@DOCSTRING(dither)
+
 Octave also provides tools to produce and work with movie frame structures.
 Those structures encapsulate the image data (@qcode{"cdata"} field) together
 with the corresponding colormap (@qcode{"colormap"} field).

etc/NEWS.11.md

@@ -123,6 +123,7 @@ Summary of important user-visible changes for version 11 (yyyy-mm-dd):
 
 * `_Exit`
 * `corrcov`
+* `dither`
 
 ### Deprecated functions, properties, and operators
 

scripts/help/__unimplemented__.m

@@ -747,7 +747,6 @@
   "discretize",
   "dissect",
   "distances",
-  "dither",
   "docsearch",
   "dragrect",
   "duration",