Membrane timers are running late

[varsill]

I have implemented an element that acts similarly to the membrane_realtimer:

• the elements starts a timer on handle_playing with :no_interval
• on handle_buffer timer's interval is calculated and set with the use of :timer_interval action
• on handle_tick, the buffer is output, and the timer's interval is set to :no_interval

I have noticed, that the timer starts running late (with latency in respect to walltime equal to ~100ms) and the compensation mechanism does not seem to take that under consideration, as the latency persists over time.
I have also experimented with using :start_timer and :stop_timer (with no luck).
It's possible that the problem would occur also with the membrane_realtimer, but I haven't checked it.
After rewriting the element so that it directly uses Process.send_after (with a similar compensation mechanism as the one introduced in membrane_core), the problem has been solved.

[varsill]

It turns out that the problem was with the handle_tick returning timer_interval: :no_interval action and then demand: ... action. The demand action was then calling handle_buffer synchronously (since there were some buffers already waiting in the input queue) and that handle_buffer was overwriting timer_interval with a particular interval. When such a chain of actions was returned by the CallbackHandler.exec_and_handle_callback in the TimerController, the Timer.tick was called unaware of the fact, that there was :no_interval in between of the actions.
It should be solved with this PR: #685