Ad is not horizontally centered in GameResults modal

[codergautam]

The ad on the Game Results or end screen isn't always horizontally centered. I've narrowed down the issue to this part of the code added in #23 :

32e0d03#diff-f2286f8bf2376694d9c87b819c26cc746048c6c82427ac6036418b9cf48389a3R56

Sometimes Adinplay serves ads smaller than the one we requested, for example a 728x90 when requesting a 970x90 ad. We need an accurate way of determining what the size of the ad being served is, so that it can be centered properly.