Added style folding optimization

This adds an optimization that removes Mapnik style rules that can never
be applied.

lib/carto/renderer.js

@@ -123,7 +123,7 @@ carto.Renderer.prototype.render = function render(m) {
         sorted = sortStyles(rules, env);
 
         for (var k = 0, rule, style_name; k < sorted.length; k++) {
-            rule = sorted[k];
+            rule = foldStyle(sorted[k]);
             style_name = l.name + (rule.attachment !== '__default__' ? '-' + rule.attachment : '');
 
             // env.effects can be modified by this call
@@ -355,6 +355,19 @@ function sortStyles(styles, env) {
     return result;
 }
 
+// Removes dead style definitions that can never be reached
+// when filter-mode="first". The style is modified in-place
+// and returned. The style must be sorted.
+function foldStyle(style) {
+    for (var i = 0; i < style.length; i++) {
+        for (var j = style.length - 1; j > i; j--) {
+            if (style[j].filters.cloneWith(style[i].filters) === null)
+                style.splice(j, 1);
+        }
+    }
+    return style;
+}
+
 /**
  * Find a rule like Map { background-color: #fff; },
  * if any, and return a list of properties to be inserted

test/rendering/style_fold.mml

@@ -0,0 +1,11 @@
+{
+  "Layer": [
+    {
+      "name": "a",
+      "id": "a"
+    }
+  ],
+  "Stylesheet": [
+    "style_fold.mss"
+  ]
+}

test/rendering/style_fold.mss

@@ -0,0 +1,14 @@
+Map { }
+#a {
+  [x = 'X'] {
+    #a {
+      line-cap: round;
+    }
+  }
+  [y = 'Y'] {
+    line-width: 2;
+    #a {
+      line-cap: round;
+    }
+  }
+}

test/rendering/style_fold.result

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE Map[]>
+<Map>
+
+
+<Style name="a" filter-mode="first">
+  <Rule>
+    <Filter>([y] = 'Y')</Filter>
+    <LineSymbolizer stroke-linecap="round" stroke-width="2" />
+  </Rule>
+  <Rule>
+    <Filter>([x] = 'X')</Filter>
+    <LineSymbolizer stroke-linecap="round" />
+  </Rule>
+</Style>
+<Layer name="a"
+>
+    <StyleName>a</StyleName>  </Layer>
+
+</Map>