Add Device::present_bound_surface API

[mrobinson]

In order to present a widget surface that is bound to a context, a user
of surfman must currently:

• Unbind the surface
• Present the surface
• Rebind the surface

On EGL platforms unbinding the surface calls:

eglMakeCurrent(egldisplay, EGL_NO_SURFACE, EGL_NO_SURFACE, egl_context);

The expected resize story for Wayland is:

1. The server calls the application's configure callback with the new
   window size.
2. The application calls wl_egl_window_resize.
3. The application waits for the next frame callback.
4. The application redraws into the surface at the new size.

Notably, wl_egl_window_resize does not resize the surface
immediately. Resizing happens later in a driver-defined way.

In Servo we handle this by:

1. Receiving the winit resize signal.
2. Unbinding the surface (calls eglMakeCurrent with NO_SURFACE
   values).
3. Resizing the surface (calls wl_egl_window_resize)
4. Rebinding the surface.
5. Waiting for the redraw signal from winit (presumably triggered by
   the Wayland frame callback).
6. Repainting the contents at the new size using OpenGL.
7. Unbinding the surface
8. Presenting the surface (calls eglSwapBuffers)
9. Rebinding the surface

Either calling eglMakeCurrent before calling eglSwapBuffers or
calling eglSwapBuffers with no surface bound, means that the driver is
failing to properly resize the surface after wl_egl_window_resize. In
general, constantly having to call eglMakeCurrent multiple times in
order to call eglSwapBuffers is both disruptive and not how typical
OpenGL applications work.

So this change adds a way to present the surface without having to first
unbind it from the context. A followup change will address resizing. It
does not make any changes to existing APIs.