Use the common helper provided by the shell-utils helper dependency,
rather than rolling our own.

This commit currently introduces no functional change to
fullscreen-shell, as the 'curtain' provided by shell-utils behaves
identically to the previous solid-color surface created by
fullscreen-shell, given the parameters provided to
weston_curtain_create().

However, now that a common weston_curtain implementation is being used
rather than an open-coded variant, future changes to the implementation
of weston_curtain will result in changes to this code called by
fullscreen-shell, although it is intended that these will not result in
any user-visible behavioural changes.

Signed-off-by: Daniel Stone <daniels@collabora.com>