window: convert shm path to toysurface

window: convert shm path to toysurface

Implement shm_surface as a sub-class of toysurface, and unify the
toysurface call sites removing most buffer type specific branching.

Do not destroy and create a surface, if the size does not change.

The resizing optimization of shm surfaces is retained, but the pool is
moved from struct window to struct shm_surface, since it does not apply
to egl_window_surface.

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