The function weston_log_ctx_compositor_destroy(), which destroys struct
weston_log_context, takes weston_compositor as argument. We may have a
weston_log_context unlinked from a weston_compositor and currently there
is no way to destroy it.
Add function weston_log_ctx_destroy(), what makes the destruction of
weston_log_context independent of weston_compositor.
With this change, one could destroy a weston_compositor and keep the
related weston_log_context (since now weston_log_context can be destroyed
without the need of a weston_compositor). But if weston_compositor gets
destroyed it's also necessary to destroy weston_log_context::global,
as the debug protocol depends on the compositor. So a listener has been
added to the destroy signal of weston_compositor.
Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>