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/CMakeLists.txt

@@ -407,6 +407,17 @@ target_include_directories(mkw_vr_first_person_tests PRIVATE "${CMAKE_CURRENT_LI
 target_compile_features(mkw_vr_first_person_tests PRIVATE cxx_std_17)
 add_test(NAME mkw_vr_first_person_tests COMMAND mkw_vr_first_person_tests)
 
+# The first-person cockpit (seated eye, wheel and handlebar geometry, level seat,
+# native wheel vertices) and hand steering (grab, turn, hand-off to the game),
+# ported from heurazy's mario-kart-wii-VR-port. All header-only.
+foreach(test_name mkw_steering_wheel_tests mkw_vr_cockpit_tests mkw_vr_hand_steering_tests)
+    string(REGEX REPLACE "^mkw_" "" test_source "${test_name}")
+    add_executable(${test_name} "${CMAKE_CURRENT_LIST_DIR}/tests/${test_source}.cpp")
+    target_include_directories(${test_name} PRIVATE "${CMAKE_CURRENT_LIST_DIR}/include")
+    target_compile_features(${test_name} PRIVATE cxx_std_17)
+    add_test(NAME ${test_name} COMMAND ${test_name})
+endforeach()
+
 # The VR controllers' Wii Remote presentation (accelerometer frame, pointer
 # raycast, debounce, button profile) is header-only for the same reason.
 add_executable(mkw_vr_wii_remote_tests "${CMAKE_CURRENT_LIST_DIR}/tests/vr_wii_remote_tests.cpp")