[ticket/10933] Initialize template context when template is constructed.

There is no apparent reason for either initializing or clearing
the context in set_style/set_custom_style.

Initially the initialization there was added in
0501640d5d, for reasons that
presently I do not see.

This permits making context property back private.

PHPBB3-10933

phpBB/includes/template/template.php

@@ -36,7 +36,7 @@ class phpbb_template
 	* Stores template data used during template rendering.
 	* @var phpbb_template_context
 	*/
-	public $context;
+	private $context;
 
 	/**
 	* Path of the cache directory for the template
@@ -95,6 +95,7 @@ class phpbb_template
 		$this->user = $user;
 		$this->locator = $locator;
 		$this->template_path = $this->locator->template_path;
+		$this->context = new phpbb_template_context();
 	}
 
 	/**