clutter-offscreen-effect: Allocate the cogl texture directly

Cogl now lazy loads the textures so we cannot rely on getting NULL
from cogl_texture_new_with_size so we have to allocate it by ourselves.

https://bugzilla.redhat.com/show_bug.cgi?id=975171
build: Enable Cogl support with Wayland

The Wayland backend is based on Cogl, so we need to turn on the
SUPPORT_COGL flag to avoid breaking the build; this always went
unnoticed because we usually build the Wayland client backend
with the X11 backend.

Reported-by: Ross Burton <ross.burton@intel.com>
(cherry picked from commit 1fb0295ba162507fb798b2b7030f0f45ff252f27)

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>