If we use the function weston_log_context_add_log_scope()
in non-core code, it's necessary to access
weston_compositor::weston_log_ctx.
This is not ideal, since the goal is to make core structs
(weston_compositor, weston_surface, weston_output, etc)
opaque.
Add function weston_compositor_add_log_scope(), so non-core
users are able to pass weston_compositor as argument instead
of weston_compositor::weston_log_ctx.
Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>