Idiomatic CoreAnimation approach

Please consider using this approach 

https://github.com/JanX2/yrk-spinning-progress-indicator-layer/tree/idiomatic-core-animation

instead of using displaylink

https://github.com/kelan/YRKSpinningProgressIndicator/compare/displaylink

This way, everything runs in the background using the GPU where possible. Using CoreAnimation directly is much more efficient. It’s also easier to maintain. 
