ios playback: initialize slider value (position) at 0 at first

Until now the slider was displaying center position before playback was
started.

playback/player/ios/GstPlay/VideoViewController.m

@@ -74,6 +74,8 @@
     
     is_local_media = [uri hasPrefix:@"file://"];
     is_playing_desired = NO;
+
+    time_slider.value = 0;
     time_slider.minimumValue = 0;
     time_slider.maximumValue = 0;
 }