class OnOutputConstraints

This event informs the client of new output constraints.

This message is ordered with respect to other output (such as output

packets, output format, output end-of-stream).

Before the first OnOutputPacket() of a stream, the server guarantees

that at least one OnOutputConstraints() and exactly one OnOutputFormat()

will be sent. The client must tolerate multiple OnOutputConstraints()

(and 1 OnOutputFormat() message) before the first output packet. As

long as the client hasn't moved to a new stream, the server won't send

another OnOutputConstraints() until after the client has configured

output buffers.

This message can be sent mid-stream by a server, when the server needs

new output buffers. The server won't generate more output until the

client has added new output buffers via SetOutputBufferPartialSettings

or AddBuffer.

The server will not send any OnOutputPacket() for this stream until

after the client has configured/re-configured output buffers.

All clients, even those which don't want to support any mid-stream

output buffer re-config or mid-stream OnOutputFormat() are required to

deal with 1..multiple OnOutputConstraints() messages before the first

output packet, and one OnOutputFormat() messages before the first output

packet.