protocol: do not change pending x and y when attaching a buffer

Attaching a buffer with interface version 5 requires clients to pass zero to x and y but it still affects the pending surface state. Attaching a buffer after a request to offset therefore sets the pending x and y to zero. The intent of version 5 was to allow exactly this sequence of requests to work so let's just make sure the protocol actually spells it out.

Signed-off-by: Sebastian Wick sebastian.wick@redhat.com