PDF: Finish support for Optional Content state change links

I implemented this 3 years ago, but for some reason forgot to commit this part ^_^

CCBUGS: 408849

generators/poppler/generator_pdf.cpp

@@ -525,7 +525,12 @@ Okular::Action* createLinkFromPopplerLink(const Poppler::Link *popplerLink, bool
 #endif
         
         case Poppler::Link::OCGState:
-            Q_UNREACHABLE();
+#ifdef HAVE_POPPLER_0_50
+            link = new Okular::BackendOpaqueAction();
+            link->setNativeId( QVariant::fromValue( static_cast<const Poppler::LinkOCGState*>( popplerLink ) ) );
+            deletePopplerLink = false;
+#endif
+        break;
     }
 
 #ifdef HAVE_POPPLER_0_64