images: adding an "each-pixel" that iterates over pixels by row.

basis/images/images.factor

@@ -167,3 +167,13 @@ PRIVATE>
 
 : set-pixel-at ( pixel x y image -- )
     [ 1 ] dip set-pixel-row-at ; inline
+
+:: each-pixel ( ... image quot: ( ... x y pixel -- ... ) -- ... )
+    image dim>> first2 :> ( width height )
+    image bytes-per-pixel :> n
+    height width [ iota ] bi@ [| y x |
+        y width * x + :> start
+        start n * :> from
+        from n + :> to
+        x y from to image bitmap>> <slice> quot call
+    ] cartesian-each ; inline