Return setseek position if one exists in get_playback_position.

(cherry picked from commit 15b8480b2c76eecb5c9c31b88828345eed6224d6)

scene/2d/audio_stream_player_2d.cpp

@@ -358,6 +358,9 @@ bool AudioStreamPlayer2D::is_playing() const {
 float AudioStreamPlayer2D::get_playback_position() {
 
 	if (stream_playback.is_valid()) {
+		if (setseek >= 0.0) {
+			return setseek;
+		}
 		return stream_playback->get_playback_position();
 	}
 

scene/3d/audio_stream_player_3d.cpp

@@ -742,6 +742,9 @@ bool AudioStreamPlayer3D::is_playing() const {
 float AudioStreamPlayer3D::get_playback_position() {
 
 	if (stream_playback.is_valid()) {
+		if (setseek >= 0.0) {
+			return setseek;
+		}
 		return stream_playback->get_playback_position();
 	}
 

scene/audio/audio_stream_player.cpp

@@ -281,6 +281,9 @@ bool AudioStreamPlayer::is_playing() const {
 float AudioStreamPlayer::get_playback_position() {
 
 	if (stream_playback.is_valid()) {
+		if (setseek >= 0.0) {
+			return setseek;
+		}
 		return stream_playback->get_playback_position();
 	}