nested: Add a renderer using subsurfaces

nested: Add a renderer using subsurfaces

Adds a second renderer implementation to the nested compositor example
that creates a subsurface for each of the client's surfaces. The
client buffers are directly attached to the subsurface using the
EGL_WL_create_wayland_buffer_from_image extension instead of blitting
them in the redraw_handler.

The new renderer is always used if the parent compositor supports the
wl_subcompositor protocol and the EGL extension is available.
Otherwise it will fall back to the blit renderer.