improve compatibility of colormap functions * autumn.m, bone.m, cool.m, copper.m, flag.m, gmap40.m, gray.m, hot.m, hsv.m, jet.m, lines.m, ocean.m, pink.m, prism.m, rainbow.m, spring.m, summer.m, winter: Always size and return 0x3 when size < 1. Return same values as Matlab when size == 1.

--- a/scripts/image/winter.m	Wed May 16 16:52:08 2012 -0400
+++ b/scripts/image/winter.m	Wed May 16 16:52:44 2012 -0400
@@ -50,7 +50,7 @@
     b = 1 - g / 2;
     map = [r, g, b];
   else
-    map = [];
+    map = zeros (0, 3);
   endif
 
 endfunction