Some downstream user might want to use the very handy comp_buffer_init()
helper to create buffers, but the latter does not cover the case when
the buffer needs to be exportable for API interoperability.
Add a new comp_buffer_init_exportable() helper to cover that case, this
is done in a way that is not particularly invasive for existing users of
comp_buffer_init(), as all the logic about the exportability is handled
by the new function.
Newer wayland protocols have a new filed in xdg_toplevel_listener which
is not initialized:
-----------------------------------------------------------------------
[215/315] Building C object src/xrt/compositor/CMakeFiles/comp_main.dir/main/comp_window_wayland.c.o
.../src/xrt/compositor/main/comp_window_wayland.c:182:1: warning: missing field 'configure_bounds' initializer [-Wmissing-field-initializers]
};
^
1 warning generated.
-----------------------------------------------------------------------
Detect the version when this is needed and initialize the field to
silence the warning.
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.
