Keep unreferenced buffers around

One problem with delaying destruction of buffer objects is that the
compositor may create a shadow that references defunct buffers.

One way to fix that issue is to immediately destroy buffers. However,
there is other way to address the issue - keep released buffers alive.

If a buffer is kept alive by the client, then it will most likely be
used again. It also simplifies buffer management.

BUG: 425233