Add DOMPDF_ENABLE_AUTOLOAD constant

Composer have changed the name of their autoloader class - now it is randomly generated each time. Therefore the original trick I added to detect `ComposerAutoloaderInit` and prevent loading of the DOMPDF autoloader no longer works. I've also been told that using `class_exists` can trigger bugs when using APC so it's probably not the best way to include php-font-lib.

Instead I've added a new constant to enable/disable the DOMPDF autoloader. That way, composer users can disable the DOMPDF autoloader and still include the `dompdf_config.inc.php` file to initialize all other defaults like so:

    define('DOMPDF_ENABLE_AUTOLOAD', false);
    require('dompdf_config.inc.php');

Obviously in future you want to completely get away from using defined constants, but this seems like the best immediate solution. Currently the only way to avoid this problem is to duplicate all the code in `dompdf_config.inc.php` and remove the autoloader bit.