wlr_scene_*_create() functions all allocate memory via calloc() and
return NULL if the allocation fails. Previously, the failures were
handled in any of 3 different ways:
- sending a wayland protocol error
- exiting labwc with an error
- segfault (no NULL check at all)
Since labwc does not attempt to survive heap exhaustion in other
allocation paths (such as `znew`), it seems more consistent to use the
same die_if_null() check used in those paths to exit with an error.
For the three most common create() functions (tree, rect, buffer),
add small lab_wlr_ wrappers to common/scene-helpers.
This is a common practice in C projects, which simply enforces that
each header must compile cleanly without implicit dependencies on
other headers (see also the previous commit).
