Prioritize sorting of hash keys when dumping PDF data.

Since only the first occurence of any given reference is dumped in full,
it's better to output Pages before Outlines, etc.

lib/PDF/Data.pm

@@ -1254,6 +1254,20 @@ sub dump_object {
   # Dump output.
   my $output = "";
 
+  # Hash key sort priority.
+  my %priority = (
+    Type           => -2,
+    Version        => -1,
+    Root           => 1,
+    Pages          => 2,
+    PageLabels     => 3,
+    Names          => 4,
+    Dests          => 5,
+    Outlines       => 6,
+    Threads        => 7,
+    StructTreeRoot => 8,
+  );
+
   # Check mode and object type.
   if ($mode eq "outline") {
     if (ref $object and $seen->{$object}) {
@@ -1266,7 +1280,7 @@ sub dump_object {
         $output = "(STREAM)";
       } else {
         $label =~ s/(?<=\w)$/->/;
-        my @keys = sort { fc($a) cmp fc($b) || $a cmp $b; } keys %{$object};
+        my @keys = sort { ($priority{$a} // 0) <=> ($priority{$b} // 0) || fc($a) cmp fc($b) || $a cmp $b; } keys %{$object};
         my $key_len = max map length $_, @keys;
         foreach my $key (@keys) {
           my $obj = $object->{$key};
@@ -1310,7 +1324,7 @@ sub dump_object {
     $seen->{$object} = $label;
     $output = "{ # $label\n";
     $label =~ s/(?<=\w)$/->/;
-    my @keys = sort { fc($a) cmp fc($b) || $a cmp $b; } keys %{$object};
+    my @keys = sort { ($priority{$a} // 0) <=> ($priority{$b} // 0) || fc($a) cmp fc($b) || $a cmp $b; } keys %{$object};
     my $key_len = max map length $_, @keys;
     foreach my $key (@keys) {
       my $obj = $object->{$key};