Fix of a possible locales mismatch during GCode export

There is a mysterious issue with GCode annotations printed with decimal
commas instead of points. tbb threads should be set to "C" locales when started,
either another thread is spawned by tbb, or someone switches the thread locales
and leaves it in the pool.

src/libslic3r/GCode.cpp

@@ -2129,13 +2129,13 @@ GCode::LayerResult GCode::process_layer(
     // add tag for processor
     gcode += ";" + GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Layer_Change) + "\n";
     // export layer z
-    char buf[64];
+    gcode += std::string(";Z:") + float_to_string_decimal_point(print_z) + "\n";
-    sprintf(buf, ";Z:%g\n", print_z);
+
-    gcode += buf;
     // export layer height
     float height = first_layer ? static_cast<float>(print_z) : static_cast<float>(print_z) - m_last_layer_z;
-    sprintf(buf, ";%s%g\n", GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Height).c_str(), height);
+    gcode += std::string(";") + GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Height)
-    gcode += buf;
+        + float_to_string_decimal_point(height) + "\n";
+
     // update caches
     m_last_layer_z = static_cast<float>(print_z);
     m_max_layer_z  = std::max(m_max_layer_z, m_last_layer_z);
@@ -3002,33 +3002,34 @@ std::string GCode::_extrude(const ExtrusionPath &path, std::string description,
     // PrusaMultiMaterial::Writer may generate GCodeProcessor::Height_Tag lines without updating m_last_height
     // so, if the last role was erWipeTower we force export of GCodeProcessor::Height_Tag lines
     bool last_was_wipe_tower = (m_last_processor_extrusion_role == erWipeTower);
-    char buf[64];
     assert(is_decimal_separator_point());
 
     if (path.role() != m_last_processor_extrusion_role) {
         m_last_processor_extrusion_role = path.role();
+        char buf[64];
         sprintf(buf, ";%s%s\n", GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Role).c_str(), ExtrusionEntity::role_to_string(m_last_processor_extrusion_role).c_str());
         gcode += buf;
     }
 
     if (last_was_wipe_tower || m_last_width != path.width) {
         m_last_width = path.width;
-        sprintf(buf, ";%s%g\n", GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Width).c_str(), m_last_width);
+        gcode += std::string(";") + GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Width)
-        gcode += buf;
+               + float_to_string_decimal_point(m_last_width) + "\n";
     }
 
 #if ENABLE_GCODE_VIEWER_DATA_CHECKING
     if (last_was_wipe_tower || (m_last_mm3_per_mm != path.mm3_per_mm)) {
         m_last_mm3_per_mm = path.mm3_per_mm;
-        sprintf(buf, ";%s%f\n", GCodeProcessor::Mm3_Per_Mm_Tag.c_str(), m_last_mm3_per_mm);
+        gcode += std::string(";") + GCodeProcessor::Mm3_Per_Mm_Tag
-        gcode += buf;
+            + float_to_string_decimal_point(m_last_mm3_per_mm) + "\n";
     }
 #endif // ENABLE_GCODE_VIEWER_DATA_CHECKING
 
     if (last_was_wipe_tower || std::abs(m_last_height - path.height) > EPSILON) {
         m_last_height = path.height;
-        sprintf(buf, ";%s%g\n", GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Height).c_str(), m_last_height);
+
-        gcode += buf;
+        gcode += std::string(";") + GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Height)
+            + float_to_string_decimal_point(m_last_height) + "\n";
     }
 
     std::string comment;