And rename it to "ensureLayerExists"

Taken from: facebook/react-native@cbf65f2

We cannot set "layer.transform" directly except from the "displayLayer:" method, because AppKit often sets it without respecting explicit values. For example, the "setFrame:" method is one spot, but there are others.

This prevents a crash when a RCTViewManager represents a NSView subclass (instead of a RCTView subclass), which requires the subclass to implement its own "transform" property and override its "displayLayer:" method.

Ensure a layer exists before setting these props.

Otherwise, the anchor point won't be set.

Otherwise, the anchorPoint may be reverted to the top-left.

The "setFrame:" method overrides our transform, so it needs to be reapplied.

- The "_backgroundColor" ivar was not being set when called with nil.

- The method assumed that the "layer" property could be nullified when "backgroundColor" is nil, which is definitely not the case.