Add "Merger > Various > Split Memory Optimized" demo

[JanSlabon]

We should add a demo, that uses a split process encapsualted in a function scope to use less memory.

/**
 * @param string $path Path of the file you want to split
 * @param callable $name A callable, that returns a path for the resulting page
 * @param int $limit Numbers of pages that should be split by an initiated document instance
 * @return void
 */
function split(string $path, callable $name, int $limit = 100)
{
    $pageNo = 1;
    $split = static function(string $path) use ($limit, $name, &$pageNo) {
        $document = \setasign\SetaPDF2\Core\Document::loadByFilename($path);
        $pageCount = $document->getCatalog()->getPages()->count();
        $document->getCatalog()->getPages()->ensureAllPageObjects();

        for (; $pageNo <= $pageCount && $limit > 0; $pageNo++) {
            $merger = new \setasign\SetaPDF2\Merger\Merger();
            $merger->addDocument($document, $pageNo);
            $merger->merge();
            $resDocument = $merger->getDocument();
            $resDocument->setWriter(new \setasign\SetaPDF2\Core\Writer\FileWriter($name($pageNo, $pageCount, $path)));
            $resDocument->save()->finish();
            $limit--;
        }

        unset($resDocument, $merger, $document);
        gc_collect_cycles();

        return $pageCount - $pageNo;
    };

    while ($split($path) > 0) {
        echo memory_get_usage(true) . PHP_EOL; // debug
    }
}

split($_GET['f'], static function (int $pageNo, int $pageCount, string $path) {
    return '_split/'. $pageNo . '.pdf';
});