generate blur frames only once on addSubview:

This PR fixes issue when blur frames re-generates every time on setting `frame` to `ANBlurredImageView` instance. For example, in UIKit animations:

```obj-c
ANBlurredImageView *blurredImageView = <...>;
[self addSubview:blurredImageView]; // from now, frames generates here

// ...

blurredImageView.frame = <...>; // does not call expensive generateBlurFramesWithCompletion:

[UIView animateWithDuration:0.25 animations:^{
    blurredImageView.frame = <...>;
}];
```