Only the camera tracking config writes and uses the version field.
Unfortunately the tracking override config is written into the same json
object while not making use of the version field.
If the last returned display time shifts backwards slightly with respect to the
last sampled display time from the compositor, the next predicted display time
will not move forward by one frame. Adding half the display period to the
comparison makes the pacing robust to this case.
Add some missing returns pointed out by -Wreturn-type:
-----------------------------------------------------------------------
[75/1571] Building C object src/xrt/auxiliary/CMakeFiles/aux_util.dir/util/u_config_json.c.o
.../src/xrt/auxiliary/util/u_config_json.c: In function ‘u_gui_state_scene_to_string’:
.../src/xrt/auxiliary/util/u_config_json.c:524:1: warning: control reaches end of non-void function [-Wreturn-type]
524 | }
| ^
-----------------------------------------------------------------------
Some variables are only used in asserts, so they may be unused
depending on the build type:
-----------------------------------------------------------------------
[68/315] Building C object src/xrt/auxiliary/CMakeFiles/aux_util.dir/util/u_sink_combiner.c.o
.../src/xrt/auxiliary/util/u_sink_combiner.c:188:11: warning: unused variable 'diff_ns' [-Wunused-variable]
int64_t diff_ns = frames[0]->timestamp - frames[1]->timestamp;
^
1 warning generated.
[205/315] Building C object src/xrt/compositor/CMakeFiles/comp_main.dir/main/comp_renderer.c.o
.../src/xrt/compositor/main/comp_renderer.c:872:17: warning: unused variable 'layer_count' [-Wunused-variable]
const uint32_t layer_count = c->base.slot.layer_count;
^
1 warning generated.
-----------------------------------------------------------------------
Mark them as XRT_MAYBE_UNUSED to fix the build warnings.
