[glass, wpilib] Rewrite Mechanism2d (#3281)

Substantially improves Mechanism2d by moving it to NetworkTables and adding
a robot API to create the mechanism elements, instead of requiring a JSON file.

Co-authored-by: Peter Johnson <johnson.peter@gmail.com>

wpilibc/src/main/native/cpp/smartdashboard/MechanismObject2d.cpp

@@ -0,0 +1,24 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "frc/smartdashboard/MechanismObject2d.h"
+
+using namespace frc;
+
+MechanismObject2d::MechanismObject2d(const wpi::Twine& name)
+    : m_name{name.str()} {}
+
+const std::string& MechanismObject2d::GetName() const {
+  return m_name;
+}
+
+void MechanismObject2d::Update(std::shared_ptr<NetworkTable> table) {
+  std::scoped_lock lock(m_mutex);
+  m_table = table;
+  UpdateEntries(m_table);
+  for (const wpi::StringMapEntry<std::unique_ptr<MechanismObject2d>>& entry :
+       m_objects) {
+    entry.getValue()->Update(m_table->GetSubTable(entry.getKey()));
+  }
+}