Follow-up to ae7d6db1d9

Exporting G-code on a worker thread did not work correctly as the worker
threads were using user's locale, not "C" locale.
The "C" locale is newly enforced to TBB worker threads by
name_tbb_thread_pool_threads_set_locale()

src/libslic3r/Thread.cpp

@@ -188,7 +188,8 @@ std::optional<std::string> get_current_thread_name()
 #endif // _WIN32
 
 // Spawn (n - 1) worker threads on Intel TBB thread pool and name them by an index and a system thread ID.
-void name_tbb_thread_pool_threads()
+// Also it sets locale of the worker threads to "C" for the G-code generator to produce "." as a decimal separator.
+void name_tbb_thread_pool_threads_set_locale()
 {
 	static bool initialized = false;
 	if (initialized)
@@ -233,6 +234,21 @@ void name_tbb_thread_pool_threads()
 				std::ostringstream name;
 		        name << "slic3r_tbb_" << range.begin();
 		        set_current_thread_name(name.str().c_str());
+		        // Set locales of the worker thread to "C".
+#ifdef _WIN32
+			    _configthreadlocale(_ENABLE_PER_THREAD_LOCALE);
+			    std::setlocale(LC_ALL, "C");
+#else
+				// We are leaking some memory here, because the newlocale() produced memory will never be released.
+				// This is not a problem though, as there will be a maximum one worker thread created per physical thread.
+				uselocale(newlocale(
+#ifdef __APPLE__
+					LC_ALL_MASK
+#else // some Unix / Linux / BSD
+					LC_ALL
+#endif
+					, "C", nullptr));
+#endif
     		}
         });
 }