[TwigBundle] moved Form extension initialization as late as possible

Because

 * it's better for performance (no need to init form templates if there is no forms)
 * right now, it crashes for all renderer except HTML (because the form templates obviously only exist for the HTML renderer)

The only other possible fix would be to force those resources to always use the HTML renderer

src/Symfony/Bundle/TwigBundle/Extension/FormExtension.php

@@ -48,8 +48,6 @@ class FormExtension extends \Twig_Extension
     public function initRuntime(\Twig_Environment $environment)
     {
         $this->environment = $environment;
-
-        $this->templates = $this->resolveResources($this->resources);
     }
 
     public function setTheme(FieldGroupInterface $group, array $resources)
@@ -94,6 +92,10 @@ class FormExtension extends \Twig_Extension
 
     public function render(FieldInterface $field, array $attributes = array())
     {
+        if (null === $this->templates) {
+            $this->templates = $this->resolveResources($this->resources);
+        }
+
         if ($field instanceof Form || get_class($field) === 'Symfony\Component\Form\FieldGroup') {
             return $this->templates['group']->getBlock('group', array(
                 'group'      => $field,
@@ -116,6 +118,10 @@ class FormExtension extends \Twig_Extension
 
     public function renderHidden(FieldGroupInterface $form)
     {
+        if (null === $this->templates) {
+            $this->templates = $this->resolveResources($this->resources);
+        }
+
         return $this->templates['hidden']->getBlock('hidden', array(
             'fields' => $form->getHiddenFields()
         ));
@@ -123,6 +129,10 @@ class FormExtension extends \Twig_Extension
 
     public function renderErrors($formOrField)
     {
+        if (null === $this->templates) {
+            $this->templates = $this->resolveResources($this->resources);
+        }
+
         return $this->templates['errors']->getBlock('errors', array(
             'errors' => $formOrField->getErrors()
         ));
@@ -130,6 +140,10 @@ class FormExtension extends \Twig_Extension
 
     public function renderLabel(FieldInterface $field, $label = null, array $attributes = array())
     {
+        if (null === $this->templates) {
+            $this->templates = $this->resolveResources($this->resources);
+        }
+
         return $this->templates['label']->getBlock('label', array(
             'id'         => $field->getId(),
             'key'        => $field->getKey(),