Add Transform2d::Inverse() (#2407)

This is useful for undoing transformations. One application my FRC team
found was converting perspective n-point data from a "camera to target"
coordinate frame transformation to a "target to camera" coordinate frame
transformation.

wpilibc/src/test/native/cpp/geometry/Transform2dTest.cpp

@@ -0,0 +1,33 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include <cmath>
+
+#include "frc/geometry/Pose2d.h"
+#include "frc/geometry/Rotation2d.h"
+#include "frc/geometry/Transform2d.h"
+#include "frc/geometry/Translation2d.h"
+#include "gtest/gtest.h"
+
+using namespace frc;
+
+static constexpr double kEpsilon = 1E-9;
+
+TEST(Transform2dTest, Inverse) {
+  const Pose2d initial{1_m, 2_m, Rotation2d(45.0_deg)};
+  const Transform2d transform{Translation2d{5.0_m, 0.0_m}, Rotation2d(5.0_deg)};
+
+  auto transformed = initial + transform;
+  auto untransformed = transformed + transform.Inverse();
+
+  EXPECT_NEAR(initial.Translation().X().to<double>(),
+              untransformed.Translation().X().to<double>(), kEpsilon);
+  EXPECT_NEAR(initial.Translation().Y().to<double>(),
+              untransformed.Translation().Y().to<double>(), kEpsilon);
+  EXPECT_NEAR(initial.Rotation().Degrees().to<double>(),
+              untransformed.Rotation().Degrees().to<double>(), kEpsilon);
+}