window: make display_create_surface() shm-only
window: make display_create_surface() shm-only

Nothing uses it to create EGL-surfaces outside of window.c. This makes
refactoring the EGL-based code easier, since we do not need to support
EGL-based Cairo surfaces without an associated struct window.

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

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