Fix for UTF-8 roundtrip verification bug.

The problem was that the round-trip verification code copies the
current state, which may contain intermediate Parser state for
multibyte characters and ANSI escape sequences.  It then applies diffs
to that copy, which may appear as badly formed input.

Also removes some dead, never-used code.

src/terminal/terminalframebuffer.cc

@@ -662,8 +662,9 @@ bool Cell::compare( const Cell &other ) const
 
   if ( !contents_match( other ) ) {
     ret = true;
-    fprintf( stderr, "Contents: %lc vs. %lc\n",
-	     debug_contents(), other.debug_contents() );
+    fprintf( stderr, "Contents: %lc (%ld) vs. %lc (%ld)\n",
+	     debug_contents(), contents.size(),
+	     other.debug_contents(), other.contents.size() );
   }
 
   if ( fallback != other.fallback ) {