simple-shm: Don't attach the buffer to the surface for every redraw
simple-shm: Don't attach the buffer to the surface for every redraw

This will allow us to change just a sub region of the buffer.

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