[video] MovieGl::getSize() returns [0,0] right after being created

If you do the following:

``` cpp
mVideo = MovieGl::create( fullPath );
auto size = mVideo->getSize();
```

`size` is [0,0], which is unexpected if you wanted to use it for laying things out. Probably because the size isn't set until the firs texture is grabbed, but I think it would be ideal if the video properties were queried to determine the size before draw time.
