Update the blur layer frame inside layoutSubviews.

I had some problems when using auto layout. The `setFrame` seems not to be called when the size of a blur view gets changed. Resizing the layer inside the `layoutSubviews` method fixed my problem.

Changing the blur layer size inside `layoutSubviews` might allow to remove the `setFrame` method completely. But I didn't do any testing and left the `setFrame` as is.