vout: samplebufferdisplay: Delete converter when display closes

Also hold converter's decoder device properly

modules/video_output/apple/VLCSampleBufferDisplay.m

@@ -62,6 +62,7 @@ static vlc_decoder_device * CVPXHoldDecoderDevice(vlc_object_t *o, void *sys)
         device->ops = &ops;
         device->type = VLC_DECODER_DEVICE_VIDEOTOOLBOX;
     }
+    vlc_decoder_device_Hold(device);
     return device;
 }
 
@@ -319,6 +320,8 @@ static void Close(vout_display_t *vd)
     VLCSampleBufferDisplay *sys;
     sys = (__bridge_transfer VLCSampleBufferDisplay*)vd->sys;
 
+    DeleteCVPXConverter(sys->converter);
+
     dispatch_async(dispatch_get_main_queue(), ^{
         if ([sys.container respondsToSelector:@selector(removeVoutSubview:)]) {
             [sys.container removeVoutSubview:sys.displayView];