Add the cockpit, steering-wheel and hand-steering building blocks

The pure pieces of the first-person cockpit and hand steering, ported from
heurazy's mario-kart-wii-VR-port: the SteeringWheel grab/turn model, the
native wheel vertex rotation, the level seat stabiliser, the seated-eye and
wheel/handlebar geometry, and the XR_FB_hand_tracking_mesh loader. Adds
openxr_driving.h, the OpenXR-free snapshot the pacing thread will publish for
the guest thread, with the hand-off rule (a held wheel replaces the left
stick's X and releases that hand's grip for the game) and the wheel's
displayed angle.

New [vr] keys: first_person_seat (cockpit), cockpit_units_per_meter (100),
steering_wheel (true), native_steering_wheel (true), hand_steering (false)
and the seven wheel_* tuning keys. Nothing reads them yet.

The fresh-config template now writes the first-person defaults the
constants hold (50 / 1.5 / 0); d86dcb0 updated the constants but not the
template.

Tests: mkw_steering_wheel_tests (the fork's), mkw_vr_cockpit_tests,
mkw_vr_hand_steering_tests.

runtime/src/vr/openxr_driving.cpp

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include "vr/openxr_driving.h"
+
+#include <mutex>
+
+namespace mkw::vr {
+// Named rather than anonymous: runtime sources are unity-built in groups.
+namespace driving_bridge {
+
+struct Published {
+    std::mutex mutex;
+    DrivingSnapshot snapshot{};
+};
+
+Published& Get() {
+    static Published published;
+    return published;
+}
+
+} // namespace driving_bridge
+
+void OpenXRPublishDriving(const DrivingSnapshot& snapshot) noexcept {
+    auto& published = driving_bridge::Get();
+    std::lock_guard lock(published.mutex);
+    published.snapshot = snapshot;
+}
+
+DrivingSnapshot OpenXRReadDriving() noexcept {
+    auto& published = driving_bridge::Get();
+    std::lock_guard lock(published.mutex);
+    return published.snapshot;
+}
+
+} // namespace mkw::vr