SceneXRender::Window uses a static XRenderPicture which it didn't clean up

Summary:
Using a static XRenderPicture results in a crash on exit as for cleanup
the already destroyed xcb_connection_t* is required.

This change ensures that the static XRenderPicture gets destroyed in the
static cleanup handler for SceneXRender::Window.

CCBUG: 363251

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D1733