compositor: offer logs via weston-debug

This registers a new weston-debug scope "log" through which one can get
live log output interspersed with possible other debugging prints. This
is implemented by passing the format and varargs received in our usual
logging entrypoints through to the debug scope as well.

Anywhere where the varargs set is used twice (once for vfprintf, another
for the debug scope), we copy the va_list in order to avoid reusing it,
which leads to memory safety issues.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Maniraj Devadoss <Maniraj.Devadoss@in.bosch.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>