This abstracts away the details of particular input devices for netplay,
and adds support for mice and (similar) lightguns. Unfortunately, in
practice, no core handles mice or lightguns in a savestate-safe way, so
they need to be used in stateless mode to be reliable, but they do work.
(1) All mode change code unified, so server mode changes and client mode
changes and announcements go through the same functions
(2) New messages which are translateable and work with multiple input
devices
The new input handling makes adding and removing players more
complicated, since data can be present that's not expected from the
connected clients list, or absent that's expected in the list but
actually shouldn't be there.
Netplay state demotions, i.e. changes from playing to spectating or
disconnected states, could cause chain disconnections of all other
clients. This was due to a bug in when MODE change messages were sent.
Clients rely on the server sending all messages in its own order, and as
a consequence, the server typically holds messages for retransmission
until they can be retransmitted at the correct time. MODE messages were
not held, so could be sent early. When they were sent early, this caused
other clients to panic and disconnect.
A smaller but much stupider secondary bug was also fixed, in which the
first connection could be dropped due simply to writing connections[0]
instead of connections[i] somewhere.
This commit adds support for temporary desync in netplay. When frontend
features that can't be truly synced, in particular rewind, are used,
netplay is momentarily disabled. As soon as the feature finished, e.g. a
rewind ending, netplay resumes with a state load. For rewind, netplay
peers won't actually experience the effect of rewind, but they will load
the rewound state.
