shell: Mark geometry dirty when we change surface alpha
shell: Mark geometry dirty when we change surface alpha

We need to recompute the opaque region when the alpha changes.

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