wayland_shm: Don't use a fixed number of buffers

wayland_shm: Don't use a fixed number of buffers

We no longer allocate 3 buffers at startup, we now allocate only as needed.

Trimming the queue will come later, as there are some situations where we
might need 3 buffers and later drop down to 2 (when on a hardware plane)

Most clients will only ever need 2 buffers, so this is a reasonable RAM
savings.