nested: Add an option to disable subsurfaces
nested: Add an option to disable subsurfaces

This adds a -b option to force the nested compositor example to use
the old blit renderer even if the appropriate extensions are
available.

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