Scale thumbnails in ffmpeg to avoid excessive memory use on UHD videos

ext/media/main.php

@@ -335,7 +335,7 @@ class Media extends Extension
             $args = [
                 escapeshellarg($ffmpeg),
                 "-y", "-i", escapeshellarg($inname),
-                "-vf", "thumbnail",
+                "-vf", "scale=$scaled_size[0]:$scaled_size[1],thumbnail",
                 "-f", "image2",
                 "-vframes", "1",
                 "-c:v", "png",
@@ -344,6 +344,8 @@ class Media extends Extension
 
             $cmd = escapeshellcmd(implode(" ", $args));
 
+            log_debug('media', "Generating thumbnail with command `$cmd`...");
+
             exec($cmd, $output, $ret);
 
             if ((int)$ret === (int)0) {