Support that clients queue up more than three buffers.

Though it is wrongful behavior on the clients part, a client should
not be allowed to crash the compositor simply by posting it an extra
buffer.

Change-Id: Ia8ba0bbfc63273b0af13b81fbb9a74328289face
Reviewed-by: Michael Brasser <michael.brasser@live.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>