Add bypass for saturate(5%) filter function. Helps issue #1299

lib/less/functions.js

@@ -96,6 +96,11 @@ tree.functions = {
         return new(tree.Dimension)(Math.round(color.luma() * color.alpha * 100), '%');
     },
     saturate: function (color, amount) {
+        // filter: saturate(3.2);
+        // should be kept as is, so check for color
+        if (!color.rgb) {
+            return null;
+        }
         var hsl = color.toHSL();
 
         hsl.s += amount.value / 100;

test/css/functions.css

@@ -26,6 +26,7 @@
   luma-cyan: 79%;
   luma-white-alpha: 50%;
   contrast-filter: contrast(30%);
+  saturate-filter: saturate(5%);
   contrast-white: #000000;
   contrast-black: #ffffff;
   contrast-red: #ffffff;

test/less/functions.less

@@ -30,6 +30,7 @@
   luma-cyan: luma(#00ffff);
   luma-white-alpha: luma(rgba(255,255,255,0.5));
   contrast-filter: contrast(30%);
+  saturate-filter: saturate(5%);
   contrast-white: contrast(#fff);
   contrast-black: contrast(#000);
   contrast-red: contrast(#ff0000);