Bit of a hacky fix for #12. It seems like the issue is caused by too
many wl_shm_pools being created and then destroyed before being flushed
to the parent compositor. Might negatively impact performance, but most
performant things shouldn't be using wl_shm anyway.
Sometimes, windows will have actually been destroyed before we even
processed the events for it, so if we try to do something like get
properties on windows (as we do whenever a window is mapped) we will get
an error from the X server, but this is fine - we can just ignore this.
Should fix#16, #20, #22, etc.
