LibVT: Don't return a history size if alternate buffer is used

The line history is unavailable if the alternate screen buffer is
currently enabled. However, since TerminalWidget uses the history size
to offset its line numbers when rendering, it will try to render
inaccessible lines once the history is not empty anymore.

Userland/Libraries/LibVT/Terminal.h

@@ -156,7 +156,7 @@ public:
         }
         m_max_history_lines = value;
     }
-    size_t history_size() const { return m_history.size(); }
+    size_t history_size() const { return m_use_alternate_screen_buffer ? 0 : m_history.size(); }
 #endif
 
     void inject_string(const StringView&);