Read wheel slip from a single source

[Sycatle]

Addresses 339 — see #339.

Behaviour is identical by construction, since the value written to the network and the value read by the physics become the same variable.

The change

The branch in glide_wheel/init.lua that already decides what the wheel reports now stores its result before sending it:

if isAsleep or not state.isOnGround then
    state.effectiveForwardSlip = 0
    state.effectiveSideSlip = 0
else
    state.effectiveForwardSlip = state.lastForwardSlip
    state.effectiveSideSlip = state.lastSideSlip
end

selfTbl.SetForwardSlip( self, state.effectiveForwardSlip )
selfTbl.SetSideSlip( self, state.effectiveSideSlip )

base_glide_car/init.lua then accumulates from state instead of the network getters.

I went this way rather than the obvious one because reading state.lastSideSlip directly is not equivalent: the network value is forced to zero while a wheel is asleep or off the ground, and state keeps its last value, so a naive substitution would feed stale slip from airborne wheels into the steering. Mirroring the condition at the consumer would have worked too, but would encode the same rule in two places.

What it buys

The averages that steer the car and shift its gears no longer travel through the presentation layer, so the wheels' network traffic can be tuned — quantised, written on change only, sent less often — without touching how the vehicle drives.

It also removes eight network variable reads per vehicle per tick, though that is incidental.

Known issues

Touches the same file as #336, so the two will conflict textually if both are taken; they are independent in substance.