shell: set black surface alpha properly in switcher_next()
shell: set black surface alpha properly in switcher_next()

Make the black surfaces semitransparent when switching surface, and
if associated fullscreen surface is chosen, make black surface opaque.

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