Add connection buffer size control
Keep client connection buffers unbound but add an API set set the max buffer size for connection buffers on the server side.
The default max buffer size is controlled via the wl_display interface while each client's connection buffer size is adjustable for finer control.
The purpose is to explicitly have larger connection buffers for specific clients such as Xwayland, or set a larger buffer size for the client with pointer focus to deal with a higher input events rate.
This is largely based on @mstoeckl work to implement unbound buffers.