Android unity (v2)

• Cleanup LVS Unity initialization. Add a new Core.Unity.cs in LVS.
• Retrieve OpenGL context in the rendering thread to support Unity's multithreading mode.
• Reverse frame only on Android in Unity startup scenes.
• Enable CI builds for all Android archs.
• Figure out how to keep Unity Editor Happy ™️ with various LVS/LibVLC binaries in the same asset.
• Make CI default to release builds for the VLC Unity plugin code.
• Re-enable Windows builds
• Update general documentation to account for android (scene scripts, readme).
• Prepare Unity store text.
• Document minimum Android version supported.
• Cleanup commits.
• Build and push a new standalone Unity Android asset.
• Build and push a new cross-platform Unity asset (Windows + Android).
• Build and push updated trial crossplatform asset on videolabs.io.