Unsurprisingly, netplay and runahead are wildly incompatible; both rely
on internal rewinding, without communicating this fact to each other.
Somewhat more surprisingly, netplay already has all the infrastructure
for negative input latency, as it's structurally the same as receiving
delayed input from a peer. This patch makes the two features
"compatible" by disabling runahead per se when netplay is active, and
using runahead's configuration to adjust netplay's own input latency
feature, which is now allowed to be negative. The effect is mostly the
same (modulo the second core support), and it doesn't confuse netplay
peers.
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.
