subsurface: Add a hook point for SUBSURFACE_COMMIT_TO_CACHE.

subsurface: Add a hook point for SUBSURFACE_COMMIT_TO_CACHE.

This patch introduces a hook point for SUBSURFACE_COMMIT_TO_CACHE.
A function given to e_comp_wl_hook_add() with this hook point will be
called when changes of subsurface is committed to cache.
It will give a chance for others to cache its changes by itself.

An implementation of tizen_viewport as an extension of wayland protocol
for tizen will use this hook point for caching its changes in a
future patch.

NOTE:
Modules which listen to this hook event should apply its cached state to
current state first when wl_surface.commit occurs. Yet when there is no
cached state at that moment, it should apply pending state.

COMMIT TO CACHE
pending -> cache

wl_surface.commit
cache ? cache -> current : pending -> current

Change-Id: I35f908881d8ff06a40d90b4d78b381d31b259ef1