[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/include/frc/smartdashboard/MechanismRoot2d.h

@@ -0,0 +1,49 @@
+// 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.
+
+#pragma once
+
+#include <memory>
+
+#include <networktables/NetworkTableEntry.h>
+#include <wpi/Twine.h>
+
+#include "MechanismObject2d.h"
+
+namespace frc {
+
+/**
+ * Root Mechanism2d node.
+ *
+ * Do not create objects of this class directly! Obtain pointers from the
+ * Mechanism2d.GetRoot() factory method.
+ */
+class MechanismRoot2d : private MechanismObject2d {
+  friend class Mechanism2d;
+  struct private_init {};
+
+ public:
+  MechanismRoot2d(const wpi::Twine& name, double x, double y,
+                  const private_init&);
+
+  /**
+   * Set the root's position.
+   *
+   * @param x new x coordinate
+   * @param y new y coordinate
+   */
+  void SetPosition(double x, double y);
+
+  using MechanismObject2d::GetName;
+
+  using MechanismObject2d::Append;
+
+ private:
+  void UpdateEntries(std::shared_ptr<NetworkTable> table) override;
+  inline void Flush();
+  double m_x;
+  double m_y;
+  nt::NetworkTableEntry m_posEntry;
+};
+}  // namespace frc