Use the pipewire ticks again as the clock source.
This was disabled because the v4l2 sources created bad ticks for the
graph. Now that this is improved we can enable the ticks again.
This has the advantage that simple audio playback does not drift
anymore. The only remaining problem would be timestamp drift or
discontinuities, which we don't handle here yet.
Make a method to get the current time to compare agains the pw_time-now
field. This is currently CLOCK_MONOTONIC but make this into a method
so that we can more easily change it later.
This was introduced in 4faa28fd96 in order to correctly map the time
from the stream. From d52df30c88 on, however, the clock switched to
monotonic time, which does not need the extra-offset.
Disable `clock_reset()` for now but leave it in place so we can easily
reenable it once we use the stream time again.
This fixes video recording in Cheese and similar apps.
Closes https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/3149
