multiplayer: hold the authority's readings between updates, and hear the brakes

Two complaints, one cause underneath them.

The brake and squeal sounds came out in shreds as a train got going. Both are
shaped from UnitBrakeForce and Vel, and both of those are results of a physics
step. An update arrives ten times a second; in the twenty-odd frames between
them the local physics carries on computing its own speed and its own brake
force for a train it has no business simulating, they fall away, and the next
update snaps them back. rsBrake pitches on Vel and rsPisk gains on the brake
force ratio, so what the ear got was that sawtooth.

So the physics outputs of somebody else's train are now put back after every
local step, not only on the frame an update lands: speed, brake force, wheel
and engine revolutions, engine power, wheelslip, sand, and the running flags of
the converter and the compressors, which the local physics kept switching off
because the conditions it evaluates are not the ones the owner is evaluating.

The relays are the same problem wearing a different hat. MainCtrlActualPos - the
notch the resistors are actually on, as opposed to the one the handle is at -
was being stepped by the local automatic notching, off its own guesses, so the
clicks either fired at the wrong moment or never fired at all. It travels now,
together with ScndCtrlActualPos and StLinFlag, which stops the local notching
inventing anything, and every relay heard here is one that operated over there.

The brake unit keeps a sound bitfield of its own, and that is where the
accelerator and the cylinder venting live. Reading it clears it, so watching it
had to be done without consuming it: TBrake grows PeekSoundFlag(), and only the
flags that have just come up count as an event - the vehicle only clears them
when it has an accelerator sound to play, so a bit that stays lit is one event,
not one per frame. RaiseSoundFlag() hands them to the far end.

Snapshot version 5.

network/snapshot.h

@@ -18,7 +18,7 @@ namespace network
 {
 
 // layout of the snapshot container. a reader refuses a blob it does not understand
-constexpr uint32_t SNAPSHOT_VERSION = 4;
+constexpr uint32_t SNAPSHOT_VERSION = 5;
 
 // the sections a snapshot is built from. a reader skips over any id it does not know,
 // which is what makes the format extensible: a peer built before a section existed can
@@ -77,4 +77,15 @@ void reset_snapshot_history();
 // is called from the vehicle's own sound handling, just before it wipes them
 void note_sound_events(std::string const &Vehicle, int Events);
 
+// same, for the events the brake unit raises. those are only cleared when the vehicle
+// happens to have an accelerator sound to play, so what is passed here is peeked rather
+// than consumed and only the flags that have just appeared are taken
+void note_brake_sound_events(std::string const &Vehicle, int Events);
+
+// re-asserts, on every frame, the physics outputs of the vehicles somebody else is
+// running. an update arrives ten times a second; between them the local physics carries
+// on computing its own speed and brake force for a train it has no business simulating,
+// and the sawtooth that produces is what the brake and squeal sounds were chasing
+void hold_remote_state();
+
 } // namespace network