Compilation error in Unit Test project on Mac when using Lottie 4.0.x version

## 🐛 Bug Report

We are currently developing an application in Xamarin Forms on MAC using Visual Studio for Mac and use the lottie animation in a .NET Standard 2.1 common project, to unit test this common project we have a separate project targeting .NET Core 3.1. Everything was ok until we updated from Lottie 3.1.0 to Lottie 4.0.4 and also tried 4.0.6. After the update we have build errors on the unit test project:

/usr/local/share/dotnet/sdk/3.1.402/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(5,5): Error NETSDK1073: The FrameworkReference 'Microsoft.WindowsDesktop.App.WPF' was not recognized (NETSDK1073) (CommonProject.Tests)
/usr/local/share/dotnet/sdk/3.1.402/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(5,5): Error NETSDK1073: The FrameworkReference 'Microsoft.WindowsDesktop.App.WindowsForms' was not recognized (NETSDK1073) (CommonProject.Tests)

### Expected behavior
No errors when building

### Reproduction steps
Create a Xamarin.Forms Shell Project
Add Lottie 4.0.6 to the .NET Standard 2.1 common project
Create a NUnit test project with type .NET Core 3.1
Add .NET Standard 2.1 common project as reference to the NET Core 3.1 project
Try to build the test project

### Configuration
You can check the project below for a faster view of the configuration and problem
[TestLottie.zip](https://github.com/Baseflow/LottieXamarin/files/5454000/TestLottie.zip)

**Version:** 4.0.6

**Platform:**
- [ ] :iphone: iOS
- [ ] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ X ] :monkey: Xamarin.Forms

Start with the TestLottie.zip reproduction, especially the CommonProject.Tests .NET Core 3.1 project and its reference to the .NET Standard 2.1 common project. Reproduce the build on Mac and inspect the Microsoft.NET.Sdk FrameworkReferenceResolution error for the WPF and Windows Forms references. Done means the test project builds without NETSDK1073 after adding Lottie 4.0.6.

Written by the indexing model from the issue text.