Raise max-image-mpixels again

PDFs are quite likely to have a lot of pixels, e.g. large high resolution scans.
250 MP is a page of A0 sized paper scanned at 400 DPI,
should be enough in most cases.

src/ocrmypdf/cli.py

@@ -404,7 +404,7 @@ Online documentation is located at:
         metavar='MPixels',
         help="Set maximum number of pixels to unpack before treating an image as a "
         "decompression bomb",
-        default=128.0,
+        default=250.0,
     )
     advanced.add_argument(
         '--pdf-renderer',