impl-node: keep separate elapsed time

Don't directly update the offset when not running. The running time
is position - offset and stays constant when not running.

Instead keep an extra elapsed variable that is updated when the state
is running. The offset is then always posision - elapsed. This is more
reliable and can compensate for jumps in the position timestamps.

Fixes #3544
See #3189

src/pipewire/impl-node.c

@@ -1322,6 +1322,7 @@ static void reset_position(struct pw_impl_node *this, struct spa_io_position *po
 
 	this->target_rate = SPA_FRACTION(1, rate);
 	this->target_quantum = quantum;
+	this->elapsed = 0;
 
 	pos->clock.rate = pos->clock.target_rate = this->target_rate;
 	pos->clock.duration = pos->clock.target_duration = this->target_quantum;
@@ -1745,8 +1746,10 @@ static inline void update_position(struct pw_impl_node *node, int all_ready, uin
 		if (all_ready)
 			a->position.state = SPA_IO_POSITION_STATE_RUNNING;
 	}
-	if (SPA_LIKELY(a->position.state != SPA_IO_POSITION_STATE_RUNNING))
-		a->position.offset += a->position.clock.duration;
+	if (SPA_LIKELY(a->position.state == SPA_IO_POSITION_STATE_RUNNING))
+		node->elapsed += a->position.clock.duration;
+
+	a->position.offset = a->position.clock.position - node->elapsed;
 }
 
 /* Called from the data-loop and it is the starting point for driver nodes.

src/pipewire/private.h

@@ -732,6 +732,7 @@ struct pw_impl_node {
 	uint64_t target_quantum;
 
 	uint64_t driver_start;
+	uint64_t elapsed;		/* elapsed time in playing */
 
 	void *user_data;                /**< extra user data */
 };