Misc fixes

• Fixes a snprintf warning in t_tracker_slam.cpp

/home/simon/src/monado/src/xrt/auxiliary/tracking/t_tracker_slam.cpp: In function ‘int t_slam_create(xrt_frame_context*, t_slam_tracker_config*, xrt_tracked_slam**, xrt_slam_sinks**)’:
/home/simon/src/monado/src/xrt/auxiliary/tracking/t_tracker_slam.cpp:1074:62: warning: ‘%zu’ directive output may be truncated writing between 1 and 20 bytes into a region of size 5 [-Wformat-truncation=]
 1074 |                 (void)snprintf(label, sizeof(label), "Camera %zu", i);
      |                                                              ^~~
In function ‘void xrt::auxiliary::tracking::slam::setup_ui(TrackerSlam&)’,
    inlined from ‘int t_slam_create(xrt_frame_context*, t_slam_tracker_config*, xrt_tracked_slam**, xrt_slam_sinks**)’ at /home/simon/src/monado/src/xrt/auxiliary/tracking/t_tracker_slam.cpp:1586:10:
/home/simon/src/monado/src/xrt/auxiliary/tracking/t_tracker_slam.cpp:1074:54: note: directive argument in the range [0, 18446744073709551614]
 1074 |                 (void)snprintf(label, sizeof(label), "Camera %zu", i);
      |                                                      ^~~~~~~~~~~~
/home/simon/src/monado/src/xrt/auxiliary/tracking/t_tracker_slam.cpp:1074:31: note: ‘snprintf’ output between 9 and 28 bytes into a destination of size 12
 1074 |                 (void)snprintf(label, sizeof(label), "Camera %zu", i);
      |                       ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• Define convert_compare_op only when used

/home/simon/src/monado/src/xrt/state_trackers/oxr/oxr_session_frame_end.c:133:1: warning: ‘convert_compare_op’ defined but not used [-Wunused-function]
  133 | convert_compare_op(XrCompareOpFB xr_compare_op)
      | ^~~~~~~~~~~~~~~~~~

• Enum values not handled in switch

/home/simon/src/monado/src/xrt/auxiliary/util/u_pretty_print.c: In function ‘u_pp_xrt_input_name’:
/home/simon/src/monado/src/xrt/auxiliary/util/u_pretty_print.c:113:9: warning: enumeration value ‘XRT_INPUT_GENERIC_FACE_TRACKING’ not handled in switch [-Wswitch]
  113 |         switch (name) {
      |         ^~~~~~
/home/simon/src/monado/src/xrt/auxiliary/util/u_pretty_print.c:113:9: warning: enumeration value ‘XRT_INPUT_HTC_EYE_FACE_TRACKING’ not handled in switch [-Wswitch]
/home/simon/src/monado/src/xrt/auxiliary/util/u_pretty_print.c:113:9: warning: enumeration value ‘XRT_INPUT_HTC_LIP_FACE_TRACKING’ not handled in switch [-Wswitch]

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- Set `XRT_FEATURE_OPENXR_LAYER_FB_IMAGE_LAYOUT` to OFF by default
- Refactor `XRT_FEATURE_OPENXR*` to use `option`