Revert back to NetVips 1.2.4 and update to NetVips Native 8.10.0.

[jsm174]

These updates are based on @kleisauke linux branch at kleisauke@c34b840

This was in support of getting Visual Pinball Engine to run on Linux: kleisauke/net-vips#90

It restores NetVips back to 1.2.4 (instead of the custom NetVips.Unity fork) and uses NetVips Native 8.10.0.

It adds RuntimeIdentifier support to the project files and updates the PluginsDeploy target to copy platform specific DLLs (managed and native) to the Plugins folder.

Regardless of the platform being built on, to force all Plugins to be updated we should run:

dotnet build -c Release -r win-x64
dotnet build -c Release -r win-x86
dotnet build -c Release -r osx-x64
dotnet build -c Release -r linux-x64

The test projects (VisualPinball.Engine.Test, and VisualPinball.Unity.Test) have been updated from net472 to netcoreapp3.1

The github workflow has been updated to use dotnet for all platforms. (Previously it was using msbuild for OSX and Linux.)

dotnet build -c Release -r osx-x64
dotnet test -r osx-x64

GetTestPath() was updated to properly access the Fixtures~ directory, when running VisualPinball.Unity.Test test cases.

Unity.Burst.dll was added to Plugins\.unity to support QuadTree in VisualPinball.Unity.Test

Finally @kleisauke was added to the authors in all projects!

[freezy]

Superseded by #170. Thanks again everyone involved!