Handle nested definition elements

Such as a clipPath or symbol contained within a defs element.

src/Svg/Document.php

@@ -31,6 +31,7 @@
 class Document extends AbstractTag
 {
     protected $filename;
+    protected $_defs_depth = 0;
     public $inDefs = false;
 
     protected $x;
@@ -81,6 +82,31 @@ public function __construct() {
 
     }
 
+    /**
+     * Increase the nesting level for defs-like elements
+     *
+     * @return int
+     */
+    public function enterDefs () {
+        $this->_defs_depth++;
+        $this->inDefs = true;
+        return $this->_defs_depth;
+    }
+
+    /**
+     * Decrease the nesting level for defs-like elements
+     *
+     * @return int
+     */
+    public function exitDefs () {
+        $this->_defs_depth--;
+        if ($this->_defs_depth < 0) {
+            $this->_defs_depth = 0;
+        }
+        $this->inDefs = ($this->_defs_depth > 0 ? true : false);
+        return $this->_defs_depth;
+    }
+
     /**
      * @return SurfaceInterface
      */
@@ -215,6 +241,7 @@ protected function before($attributes)
 
     public function render(SurfaceInterface $surface)
     {
+        $this->_defs_depth = 0;
         $this->inDefs = false;
         $this->surface = $surface;
 
@@ -258,7 +285,7 @@ private function _tagStart($parser, $name, $attributes)
 
         switch (strtolower($name)) {
             case 'defs':
-                $this->inDefs = true;
+                $this->enterDefs();
                 return;
 
             case 'svg':
@@ -328,7 +355,7 @@ private function _tagStart($parser, $name, $attributes)
                 break;
 
             case 'symbol':
-                $this->inDefs = true;
+                $this->enterDefs();
                 $tag = new Symbol($this, $name);
                 break;
 
@@ -381,11 +408,11 @@ function _tagEnd($parser, $name)
         $tag = null;
         switch (strtolower($name)) {
             case 'defs':
-                $this->inDefs = false;
+                $this->exitDefs();
                 return;
 
             case 'symbol':
-                $this->inDefs = false;
+                $this->exitDefs();
                 $tag = array_pop($this->stack);
                 break;