perf: optimize media item cache key generation

closes #974 (`wp_rand()` WP 6.1 bug on 32-bit platforms)

controller.php

@@ -1180,7 +1180,7 @@ private function cache_media_item( $uri, $type, $internal, $props = null ) {
 		if ( null === $key ) {
 			// Generate Unique ID.
 			do {
-				$key = (string) wp_rand();
+				$key = mt_rand();
 			} while ( isset( $this->media_items_raw['props'][ $key ] ) );
 			// Build properties object.
 			$i = $this->media_item_template;