Check if CGContextBeginTransparencyLayer can avoid the 2-step rendering

See [Quartz 2D PG: Transparency Layers](https://developer.apple.com/library/ios/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_trans_layers/dq_trans_layers.html)

Using `CGContextBeginTransparencyLayer`/`CGContextEndTransparencyLayer` may be a solution to avoid doing a 2-step rendering (first tinting, then do the final rendering), by using layers instead of generating a first `UIImage` then using it to generate the second, final image.

This is only a hypothesis and some stuff to experiment — maybe that won't do it, but it's worth testing and digging a bit to check if that may be interesting.