Add test validating that images in nested XObjects are included in optimization

The sample file (nested-images.pdf) includes a pdf with an image that is
nested within an XObject within an XObject in the Resources dict of the
only page. These images were ignored in prior versions of qpdf.

qpdf/qtest/image-optimization.test

@@ -33,6 +33,8 @@ my @image_opt = (
     ['large-inline-image', 'inline-images-keep-all', '--keep-inline-images'],
     ['unsupported-optimization', 'unsupported',
      '--oi-min-width=0 --oi-min-height=0 --oi-min-area=0'],
+    ['nested-images', 'nested-images',
+     '--oi-min-width=0 --oi-min-height=0 --oi-min-area=0']
     );
 
 my $n_tests = 2 * scalar(@image_opt);

qpdf/qtest/qpdf/optimize-images-nested-images-json.out

@@ -0,0 +1,18 @@
+{
+  "version": 2,
+  "parameters": {
+    "decodelevel": "generalized"
+  },
+  "pages": [
+    {
+      "contents": [
+        "4 0 R"
+      ],
+      "images": [],
+      "label": null,
+      "object": "3 0 R",
+      "outlines": [],
+      "pageposfrom1": 1
+    }
+  ]
+}

qpdf/qtest/qpdf/optimize-images-nested-images.out

@@ -0,0 +1,2 @@
+qpdf: image /X1 on page 1: optimizing image reduces size from 2628 to ...
+qpdf: wrote file a.pdf