For customizing back button image we use platform native functionality that is: `setBackIndicatorImage` on iOS and `setHomeAsUpIndicator` on Android.
The reason we don't do that just by setting left item is that we get a couple of things for free such as handling RTL properly, working accessibility features, handling prop for hiding back button and a couple more.
Unfortunately there are some downsides to that approach too. We need to install the back button as an Image component from the JS side, and the extract the image payload on the native side to set it with the navigator. This is specifically problematic in DEV mode where images are loaded asynchronously over HTTP from the packager. In order for that to work we had to employ a few hacks (more comments on that in the code).
There was a bug in PR that introduces enableScreens method to replace useScreens. The bug was that the method did not end up being exported (we use module.exports and not export syntax). On top of that I'm adding a deprecation warning to useScreens method as it interferes with some react hooks tooling.
This wants to be a small PR to improve the usability of the library, based on this conversation: https://twitter.com/grifotv/status/1127847192067215360.
Since the release of RNS there has been a new major player in the React game: hooks. And sadly `useScreens` recalls too closely Hooks, and this can lead to misunderstanding.
Changing it to `enableScreens` will make the difference clear, but at the same time it will be a BREAKING CHANGE for everyone using the lib.
So if we prefer to keep it around as useScreens I'm ok with it too, and we can close this.
Also, I did some tweaks to the README + fix some typos.
This change adds an ability for screen container on Android to apply the correct mechanism for transparent layer blending. This is specifically important as screens are usually a complex views that may displays many layers and while transitioning often opacity is used to animate these. When we detect screen transitioning we (a) turn on offscreen alpha compositing (which makes the opacity being applied for the whole screen layer at once instead of making all the children semi-transparent) and also (b) turn on hardware layer that makes offscreen compositing render to GPU (which is both faster and consumes only GPU memory).
In addition to that change we also need to disable wrapping Screen's children with View, as in such a case opacity is applied on the underlying View instead of a Screen. That workaround has been added because of a bug in Animated library and fixed in RN 0.57+
