Expose two easy-to-use wrappers around `SurfmanRenderingContext` that
make the API simpler to use:
- `WindowRenderingContext`: This `RenderingContext` is a newtype around
`SurfmanRenderingContext` takes a `raw-window-handle` display and window
and creates a full window rendering context.
- `SoftwareRenderingContext`: is wraps `SurfmanRenderingContext` and
adds a swap chain in order to expose a software GL rendering context.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>