A general clean-up of the CoreGraphics implementation, instead of creating a new NSView and rendering into the layer there, we create a sublayer. See the code comments, and also gfx-rs/wgpu#6107 for some of the design decisions around this.

This is also a step forwards for making softbuffer usable from a separate thread on Apple platforms, as CALayer is fully thread safe (whereas NSView/UIView aren't).

Finally, it removes our dependence on having access to NSView, paving the way for a future where raw-window-handle may only provide CALayer.

Fixes #43.