Performance/reliability improvement for leaving unrecognised tags.

error_behaviour = self::LEAVE is the default behaviour. In this case,
we don’t even need to bother recognising such tags. Rather than
replacing with marker images and re-inserting the original text after
we’re done, we can leave them alone.

This should make the code faster and more reliable.

parsers/ShortcodeParser.php

@@ -321,6 +321,17 @@ class ShortcodeParser extends Object {
 			}
 		}
 
+		// Step 3: remove any tags that don't have handlers registered
+		// Only do this if self::$error_behavior == self::LEAVE
+		// This is optional but speeds things up.
+		if(self::$error_behavior == self::LEAVE) {
+			foreach($tags as $i => $tag) {
+				if(empty($this->shortcodes[$tag['open']])) {
+					unset($tags[$i]);
+				}
+			}
+		}
+
 		return array_values($tags);
 	}