compositor: set and use the presentation clock everywhere

Add presentation clock setters that verify the given clock actually
works. Offer an automatic choice of a software fallback clock, when a
backend has to always use clock_gettime() to approximate the
presentation time.

The DRM backend already queried the DRM about the clock id, just let the
DRM backend set the presentation clock from that.

For all other backends which do not get a timestamp from the driver,
call the software clock setter to choose a suitable clock.

Report the chosen clock via presentation.clock_id event to clients.

In finish_frame(), upgrade the argument from uint32_t milliseconds to
struct timespec which can accurately hold the presentation clock values.
This will be needed when weston_output_finish_frame() starts to send out
presentation_feedback.presented events.

While at it, replace gettimeofday() calls with clock_gettime() using the
chosen presentation clock, so we manufacture presentation timestamps
from the presentation clock when the gfx drivers cannot give us a proper
timestamp.

Rpi patch is more verbose due to not having the compositor pointer
available in rpi_flippipe_update_complete(). Explicitly carry the clock
id with flippipe so it is available in the thread.

Changes in v4:

* rpi debug build fix

v4 Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v3 Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>