Background

The Windows embedder uses EGL to interface with OpenGL. It has a single mega-type AngleSurfaceManager that contains most of the EGL logic.

This is problematic as the multi-window work uses EGL heavily to manage and present to windows' surfaces.

Work

At a high level:

1. Split the AngleSurfaceManager into egl::Surface and egl::Context types
2. Make the FlutterWindowView own its egl::Surface