The Windows embedder has three ways to get an `HWND`:
1. `GetWindowHandle` which returns the `HWND`
2. `GetPlatformWindow` which returns the `HWND` wrapped as a `PlatformWindow`
3. `GetRenderTarget` which returns the `HWND` wrapped as a `RenderTarget`
These abstractions are no longer useful now that we removed the UWP embedder. This change removes `PlatformWindow` and `RenderTarget` and uses `HWND` directly.
This change is a refactoring with no semantic changes.
e18aa559bd