kwinglutils: Assert glFenceSync

This really should not fail unless we did something seriously wrong
on our end, such as changing GL context during paintScreen.

If we add an invalid `GLSync` to the queue it can lead to very hard
to debug crashes in seemingly unrelated parts of the rendering
process, when the queue is drained, potentially seconds after the
actual failure that occurred here.

Signed-off-by: Eike Hein <eike.hein@mbition.io>