Refactor pt. 2: route player transport controls+callbacks through PlaybackManager

backend/mpmedia_other.go

@@ -4,11 +4,9 @@ package backend
 
 import (
 	"errors"
-
-	"github.com/dweymouth/supersonic/player/mpv"
 )
 
-func InitMPMediaHandler(player *mpv.Player, playbackManager *PlaybackManager, artURLLookup func(trackID string) (string, error)) error {
+func InitMPMediaHandler(playbackManager *PlaybackManager, artURLLookup func(trackID string) (string, error)) error {
 	// MPMediaHandler only supports macOS.
 	return errors.New("unsupported platform")
 }