Function XdgSurface.ackConfigure

When a configure event is received, if a client commits the surface in response to the configure event, then the client must make an ack_configure request sometime before the commit request, passing along the serial of the configure event.

For instance, for toplevel surfaces the compositor might use this information to move a surface to the top left only when the client has drawn itself for the maximized or fullscreen state.

If the client receives multiple configure events before it can respond to one, it only has to ack the last configure event.

A client is not required to commit immediately after sending an ack_configure request - it may even ack_configure several times before its next surface commit.

A client may send multiple ack_configure requests before committing, but only the last request sent before a commit indicates which configure event the client really is responding to.