[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/Mechanism2d.cpp

@@ -0,0 +1,55 @@
+// 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/Mechanism2d.h"
+
+#include <cstdio>
+
+#include "frc/smartdashboard/SendableBuilder.h"
+
+using namespace frc;
+
+static constexpr char kBackgroundColor[] = "backgroundColor";
+static constexpr char kDims[] = "dims";
+
+Mechanism2d::Mechanism2d(double width, double height,
+                         const Color8Bit& backgroundColor)
+    : m_width{width}, m_height{height} {
+  SetBackgroundColor(backgroundColor);
+}
+
+MechanismRoot2d* Mechanism2d::GetRoot(wpi::StringRef name, double x, double y) {
+  auto& obj = m_roots[name];
+  if (obj) {
+    return obj.get();
+  }
+  obj = std::make_unique<MechanismRoot2d>(name, x, y,
+                                          MechanismRoot2d::private_init{});
+  if (m_table) {
+    obj->Update(m_table->GetSubTable(name));
+  }
+  return obj.get();
+}
+
+void Mechanism2d::SetBackgroundColor(const Color8Bit& color) {
+  std::snprintf(m_color, sizeof(m_color), "#%02X%02X%02X", color.red,
+                color.green, color.blue);
+  if (m_table) {
+    m_table->GetEntry(kBackgroundColor).SetString(m_color);
+  }
+}
+
+void Mechanism2d::InitSendable(SendableBuilder& builder) {
+  builder.SetSmartDashboardType("Mechanism2d");
+  m_table = builder.GetTable();
+
+  m_table->GetEntry(kDims).SetDoubleArray({m_width, m_height});
+  m_table->GetEntry(kBackgroundColor).SetString(m_color);
+
+  std::scoped_lock lock(m_mutex);
+  for (const auto& entry : m_roots) {
+    const auto& root = entry.getValue().get();
+    root->Update(m_table->GetSubTable(entry.getKey()));
+  }
+}