Gallium: Fix shared memory segment leak

What does this MR do and why?

Commit abe6d750 caused shared memory segments to be leaked. We need to mark shared memory segments for deletion upon construction.

Fixes: abe6d750 xlib: fix glXDestroyContext in Gallium frontends

Closes: #9425 (closed)

Reviewed-by: Adam Jackson ajax@redhat.com