Simulation GUI: Add 2D field view (#2261)

The field image and robot image can be loaded or just a wireframe used.
The robot can be moved and rotated with a mouse click + drag.
The robot position is settable in robot code via the Field2d class.

wpilibc/src/main/native/include/frc/simulation/Field2d.h

@@ -0,0 +1,70 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2020 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <hal/SimDevice.h>
+#include <units/units.h>
+
+#include "frc/geometry/Pose2d.h"
+#include "frc/geometry/Rotation2d.h"
+
+namespace frc {
+
+/**
+ * 2D representation of game field (for simulation).
+ *
+ * In non-simulation mode this simply stores and returns the robot pose.
+ *
+ * The robot pose is the actual location shown on the simulation view.
+ * This may or may not match the robot's internal odometry.  For example, if
+ * the robot is shown at a particular starting location, the pose in this
+ * class would represent the actual location on the field, but the robot's
+ * internal state might have a 0,0,0 pose (unless it's initialized to
+ * something different).
+ *
+ * As the user is able to edit the pose, code performing updates should get
+ * the robot pose, transform it as appropriate (e.g. based on simulated wheel
+ * velocity), and set the new pose.
+ */
+class Field2d {
+ public:
+  Field2d();
+
+  /**
+   * Set the robot pose from a Pose object.
+   *
+   * @param pose 2D pose
+   */
+  void SetRobotPose(const Pose2d& pose);
+
+  /**
+   * Set the robot pose from x, y, and rotation.
+   *
+   * @param x X location
+   * @param y Y location
+   * @param rotation rotation
+   */
+  void SetRobotPose(units::meter_t x, units::meter_t y, Rotation2d rotation);
+
+  /**
+   * Get the robot pose.
+   *
+   * @return 2D pose
+   */
+  Pose2d GetRobotPose();
+
+ private:
+  Pose2d m_pose;
+
+  hal::SimDevice m_device;
+  hal::SimDouble m_x;
+  hal::SimDouble m_y;
+  hal::SimDouble m_rot;
+};
+
+}  // namespace frc