Better support for logger copy with backtrace

include/spdlog/details/backtracer.h

@@ -25,7 +25,11 @@ public:
         : n_messages_{n_messages}, messages_{n_messages}
     {}
 
-    size_t n_messages()
+    backtracer(const backtracer &other):
+        n_messages_{other.n_messages_}, messages_{other.messages_}
+    {}
+
+    size_t n_messages() const
     {
         return n_messages_;
     }

include/spdlog/details/log_msg_buffer.h

@@ -32,6 +32,14 @@ public:
         update_string_views();
     }
 
+    log_msg_buffer(const log_msg_buffer &other)
+            : log_msg{other}
+    {
+        buffer.append(logger_name.begin(), logger_name.end());
+        buffer.append(payload.begin(), payload.end());
+        update_string_views();
+    }
+
     log_msg_buffer(log_msg_buffer &&other)
         : log_msg{std::move(other)}
         , buffer{std::move(other.buffer)}