clutter-offscreen-effect: Disable if no texture

clutter-offscreen-effect: Disable if no texture

If texture allocation fails (e.g. on an old GPU with size limit 2048)
then `update_fbo` would return `FALSE` but leaves `priv->offscreen`
as non-NULL. So the next paint will try to use the offscreen with a
`NULL` texture and crashes. The solution is simply to ensure that
`priv->offscreen` is NULL if there is no `priv->texture`, so the default
(non-offscreen) paint path gets used instead.

Bug reported and fix provided by Gert van de Kraats.

Origin: https://gitlab.gnome.org/GNOME/mutter/commit/f31cf0c3e
Bug-Ubuntu: https://launchpad.net/bugs/1795774
Applied-upstream: 3.30.3

Gbp-Pq: Name clutter-offscreen-effect-Disable-if-no-texture.patch