fix performance in convert engine: exif orientation

In other calls to the ImageMagick commandline tools, the suffix "[0]"
is added so it only looks at the first page if it's a multi-page document
such as a PDF.

For some reason, this is missing in the `_get_exif_orientation()` method,
and may cause very long runtimes when dealing with large documents (several hundred
pages of PDF).

sorl/thumbnail/engines/convert_engine.py

@@ -110,7 +110,8 @@ def is_valid_image(self, raw_data):
 
     def _get_exif_orientation(self, image):
         args = settings.THUMBNAIL_IDENTIFY.split()
-        args.extend(['-format', '%[exif:orientation]', image['source']])
+        image_param = f'{image['source']}[0]'
+        args.extend(["-format", "%[exif:orientation]", image_param])
         p = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
         p.wait()
         result = p.stdout.read().strip()