AppleTV support + improvements for iOS backend (#14196)

* Add initial metal support for iOS

* Make iOS backend compilable with TVOS and MacCatalyst

* Enable nullable and fix almost all warnings in iOS project

* Implement keyboard and remote (+touch) input on iOS and tvOS

* Disable Metal for now

* Fix warning

* Add tvOS target for Avalonia.iOS

* Include tvos workloads

* Don't use WebKit on tvOS samples

* Extend StandardRuntimePlatform with more apple platforms

* Handle mouse and stylus for iOS backend as well

* And IntermediatePoints

* Fix compatibility warnings

* Respect t.MaximumPossibleForce

* Make ControlCatalog.IOS work with both iOS and tvOS

* Exclude compiled files using csproj instead of #if directives

* Make Metal rendering actually usable

* No need for buffer.WaitUntilCompleted on iOS here

azure-pipelines.yml

@@ -171,7 +171,7 @@ jobs:
     displayName: 'Install Workloads'
     inputs:
       script: |
-       dotnet workload install android ios wasm-tools wasm-experimental
+       dotnet workload install android ios tvos wasm-tools wasm-experimental
 
   - task: PowerShell@2
     displayName: 'Install Tizen Workload'