[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/MechanismLigament2d.h

@@ -0,0 +1,84 @@
+// 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 <units/angle.h>
+
+#include "frc/smartdashboard/MechanismObject2d.h"
+#include "frc/util/Color8Bit.h"
+
+namespace frc {
+
+/**
+ * Ligament node on a Mechanism2d.
+ *
+ * @see Mechanism2d
+ */
+class MechanismLigament2d : public MechanismObject2d {
+ public:
+  MechanismLigament2d(const wpi::Twine& name, double length,
+                      units::degree_t angle, double lineWidth = 6,
+                      const frc::Color8Bit& color = {235, 137, 52});
+
+  /**
+   * Set the ligament color.
+   *
+   * @param color the color of the line
+   */
+  void SetColor(const frc::Color8Bit& color);
+
+  /**
+   * Set the ligament's length.
+   *
+   * @param length the line length
+   */
+  void SetLength(double length);
+
+  /**
+   * Get the ligament length.
+   *
+   * @return the line length
+   */
+  double GetLength();
+
+  /**
+   * Set the ligament's angle relative to its parent.
+   *
+   * @param degrees the angle
+   */
+  void SetAngle(units::degree_t angle);
+
+  /**
+   * Get the ligament's angle relative to its parent.
+   *
+   * @return the angle
+   */
+  double GetAngle();
+
+  /**
+   * Set the line thickness.
+   *
+   * @param weight the line thickness
+   */
+  void SetLineWeight(double lineWidth);
+
+ protected:
+  void UpdateEntries(std::shared_ptr<NetworkTable> table) override;
+
+ private:
+  void Flush();
+  double m_length;
+  nt::NetworkTableEntry m_lengthEntry;
+  double m_angle;
+  nt::NetworkTableEntry m_angleEntry;
+  double m_weight;
+  nt::NetworkTableEntry m_weightEntry;
+  char m_color[10];
+  nt::NetworkTableEntry m_colorEntry;
+};
+}  // namespace frc