If color buffer is locked, do not set its wayland buffer to NULL;
otherwise it can not be freed later.

Rather, flag it in order to destroy it later on the release event.

v2: instruct release event to unlock only or free wl_buffer too (Daniel)

This also fixes dEQP-EGL.functional.swap_buffers_with_damage.* tests.

CC: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Daniel Stone <daniels@collabora.com>