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.

mchalupa/weston,kwm81/weston,eyolfson/weston,Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,krezovic/weston,udoprog/weston,giucam/weston,udoprog/weston,xorgy/weston,Fantu/compositor-spice,Fantu/compositor-spice,xorgy/weston,krezovic/weston,mchalupa/weston,sir-murray/weston,giucam/weston,Gnurou/weston,sir-murray/weston,jonnylamb/weston,Tarnyko/weston-xdg_surface_present,eyolfson/weston,jonnylamb/weston,kwm81/weston,Gnurou/weston