bin: fix handling for --output & --format with LTL outputs

* bin/common_output.cc: Set the output stream for LTL formats.

NEWS

@@ -8,6 +8,9 @@ New in spot 2.11.5.dev  (not yet released)
     where thousands of different filenames can be created failed with
     "Too many open files".  (Issue #534)
 
+  - Using --format=... on a tool that output formulas would force
+    the output on standard output, even when --output was given.
+
 
 New in spot 2.11.5  (2023-04-20)
 

bin/common_output.cc

@@ -224,7 +224,7 @@ namespace
     }
   };
 
-  class formula_printer final: protected spot::formater
+  class formula_printer final: public spot::formater
   {
   public:
     formula_printer(std::ostream& os, const char* format)
@@ -392,6 +392,7 @@ output_formula(std::ostream& out,
   else
     {
       formula_with_location fl = { f, filename, linenum, prefix, suffix };
+      format->set_output(out);
       format->print(fl, ptimer);
     }
 }