wayland: Refactor ClientBuffer
ClientBufferIntegration was introduced to work around the fact that some client buffers are not created by us.
In addition to that, DisplayPrivate::registerClientBuffer() was introduced to register client buffers created by us.
This change get rids of the ClientBufferIntegration type and further simplifies the ClientBuffer type to make wayland-specific client buffer code less messier.
Edited by Vlad Zahorodnii