egl/wayland: fix glthread deadlocks

We need to make sure that glthread is idle before using wl_* functions or they might be used from 2 threads at the same time.

Thanks to @deltib for the investigation of this issue.

Fixes: 58f90fd0 ("egl/wayland: fix glthread crashes") Closes: #7624 (closed) Closes: #8136 (closed)