Close the video buffer after using it in iOS

If the buffer is not closed, I believe a memory leak is occurring, and GC runs frequently (once every a few seconds in the device I tested, iPhone 7).

Empirically, I tried using "close" and also "release". The first one did the trick.

This change is also based on the documentation that mentions the buffer should be released after use: https://developer.apple.com/documentation/avfoundation/avplayeritemvideooutput/1386148-copypixelbufferforitemtime#return_value

gdx-video-robovm/src/com/badlogic/gdx/video/VideoPlayerIos.java

@@ -236,6 +236,7 @@ public synchronized boolean update () {
 			return false;
 		}
 		updateTextureFromBuffer(buffer);
+		buffer.close();
 		return true;
 	}