Add more playback rates for MPV

Model/Player/Backends/AVPlayerBackend.swift

@@ -21,6 +21,14 @@ final class AVPlayerBackend: PlayerBackend {
     var stream: Stream?
     var video: Video?
 
+    var suggestedPlaybackRates: [Double] {
+        [0.5, 0.67, 0.8, 1, 1.25, 1.5, 2]
+    }
+
+    func canPlayAtRate(_ rate: Double) -> Bool {
+        suggestedPlaybackRates.contains(rate)
+    }
+
     var currentTime: CMTime? {
         avPlayer.currentTime()
     }