Mark VD_AGENT_CLIPBOARD_MAX as deprecated

There is no use for those 2 values, and the default limit would be quite
wrong.

Put them in a deprecated block. If someone uses them, he will have to
add -DSPICE_DEPRECATED, or just fix the code.
vdagent: add max-clipboard message

Add an optional message sent by the client to ask the agent not to send
clipboard data bigger than a certain size, in bytes.  The message can be
sent if the agent supports the capability MAX_CLIPBOARD, at any time.

The agent is free to ignore or forget the value after a restart or a
disconnection, but a bigger message might be discarded when received on
client side, resulting in bandwidth waste.
Add support for the Opus codec

Signed-off-by: Jeremy White <jwhite@codeweavers.com>