LibPDF: Make encrypted object streams work

There were two problems:
1. parse_compressed_object_with_index() parses indirect objects
   without going through Parser::parse_indirect_value(), so
   push_reference() / pop_reference() weren't called.
   Manually call them, both for the indirect object containing
   the object stream and for the indirect object within the
   object stream.
2. The indirect object within the object stream got decrypted
   twice: Once when the object stream data itself got decrypted,
   and then incorrectly a second time when the object data within
   the stream was read. To fix, disable encryption while parsing
   object stream data (since it's already decrypted).

The test is from http://opf-labs.org/format-corpus/pdfCabinetOfHorrors/
which according to readme.md at the same location is CC0.

Userland/Libraries/LibPDF/Parser.h

@@ -62,10 +62,15 @@ public:
         m_enable_filters = enabled;
     }
 
-protected:
+    void set_encryption_enabled(bool enabled)
+    {
+        m_enable_encryption = enabled;
+    }
+
     void push_reference(Reference const& ref) { m_current_reference_stack.append(ref); }
     void pop_reference() { m_current_reference_stack.take_last(); }
 
+protected:
     Error error(
         DeprecatedString const& message
 #ifdef PDF_DEBUG