Added the function xmlSchemaValidateOneElement to the XML Schema

* xmlschemas.c include/libxml/xmlschemas.h: Added the function
  xmlSchemaValidateOneElement to the XML Schema validation API,
  as proposed by Jeffrey Fink - see bug # 152073.
diff --git a/xmlschemas.c b/xmlschemas.c
index ce4b558..12cc8c1 100644
--- a/xmlschemas.c
+++ b/xmlschemas.c
@@ -14276,6 +14276,28 @@
 }
 
 /**
+ * xmlSchemaValidateOneElement:
+ * @ctxt:  a schema validation context
+ * @elem:  an element node
+ *
+ * Validate a branch of a tree, starting with the given @elem.
+ *
+ * Returns 0 if the element and its subtree is valid, a positive error 
+ * code number otherwise and -1 in case of an internal or API error.
+ */
+int
+xmlSchemaValidateOneElement(xmlSchemaValidCtxtPtr ctxt, xmlNodePtr elem)
+{
+    if ((ctxt == NULL) || (elem == NULL))
+	return (-1);
+
+    ctxt->doc = elem->doc;
+    ctxt->err = 0;
+    ctxt->nberrors = 0;
+    return (xmlSchemaValidateElement(ctxt, elem));
+}
+
+/**
  * xmlSchemaValidateDocument:
  * @ctxt:  a schema validation context
  * @doc:  a parsed document tree