MDL-64431 assign: Ensure grading view displays PDF annotations.

When displaying submitted files in assignment grading view, run
all PDFs through Ghostscript to merge annotation layers with
content, so that the annotations display correctly.

mod/assign/feedback/editpdf/classes/pdf.php

@@ -604,36 +604,22 @@ public static function ensure_pdf_compatible(\stored_file $file) {
     }
 
     /**
-     * Check to see if PDF is version 1.4 (or below); if not: use ghostscript to convert it
+     * Use ghostscript to convert PDFs of version 1.4 or less,
+     * and PDFs containing annotation layers into single layer PDFs.
      *
      * @param   string $tempsrc The path to the file on disk.
      * @return  string path to copy or converted pdf (false == fail)
      */
     public static function ensure_pdf_file_compatible($tempsrc) {
         global $CFG;
 
-        $pdf = new pdf();
-        $pagecount = 0;
-        try {
-            $pagecount = $pdf->load_pdf($tempsrc);
-        } catch (\Exception $e) {
-            // PDF was not valid - try running it through ghostscript to clean it up.
-            $pagecount = 0;
-        }
-        $pdf->Close(); // PDF loaded and never saved/outputted needs to be closed.
-
-        if ($pagecount > 0) {
-            // PDF is already valid and can be read by tcpdf.
-            return $tempsrc;
-        }
-
         $temparea = make_request_directory();
         $tempdst = $temparea . "/target.pdf";
 
         $gsexec = \escapeshellarg($CFG->pathtogs);
         $tempdstarg = \escapeshellarg($tempdst);
         $tempsrcarg = \escapeshellarg($tempsrc);
-        $command = "$gsexec -q -sDEVICE=pdfwrite -dBATCH -dNOPAUSE -sOutputFile=$tempdstarg $tempsrcarg";
+        $command = "$gsexec -q -sDEVICE=pdfimage24 -dBATCH -dNOPAUSE -sOutputFile=$tempdstarg $tempsrcarg";
         exec($command);
         if (!file_exists($tempdst)) {
             // Something has gone wrong in the conversion.