This PR implements `FlutterMetalLayer`, a drop-in (as far as Flutter is
concerned) replacement for `CAMetalLayer`. The biggest difference is
that `FlutterMetalLayer` can present frames from background thread
within a `CATransaction`.
`FlutterMetalLayer` is disabled by default. To opt-in, add the following
item to `Info.plist`:
```xml
<key>FLTUseFlutterMetalLayer</key>
<true/>
```
The performance seems quite good, consistent 120hz on iPhone 13 Pro.
Benefits
- presenting with transaction from background thread, which, down the
line, would allow for platform views without thread merging.
- fine control over how the surface is displayed - we can display single
surface on multiple `CALayers`, each showing different part, allowing
for performant implementation of unobstructed platform views.
Drawbacks
- this not being a metal layer makes working with metal instrument tools
more awkward
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