generate blur frames only once on addSubview:

[podkovyrin]

This PR fixes issue when blur frames re-generates every time on setting frame to ANBlurredImageView instance. For example, in UIKit animations:

ANBlurredImageView *blurredImageView = <...>;
[self addSubview:blurredImageView]; // from now, frames generates here

// ...

blurredImageView.frame = <...>; // does not call expensive generateBlurFramesWithCompletion:

[UIView animateWithDuration:0.25 animations:^{
    blurredImageView.frame = <...>;
}];