Accidentally setting engine method breaks engine until matron restart

[rwhaling]

While working through the studies, I accidentally ran engine.cutoff = 100 in a script, which I rapidly fixed.

However, that created a numeric field named cutoff in the engine table, which means that when I correctly ran engine.cutoff(100), I got a attempt to call a number value (field 'cutoff') error.

Surprisingly - the error persisted even after removing the bad assignment, since the engine table itself does not appear to be re-initialized upon script load - I had to restart . Huge thanks to @catfact for figuring this out.

After a bit more testing, I've confirmed that this issue persists even after loading a different script, that uses a different engine - I believe due to the way the engine object itself persists across loading/unloading particular engines

I think this is a pretty big potential footgun for Lua novices (like me), so worth fixing. I'm still wrapping my head around how script-loading works, but I think two ways to fix would be:

1. Disable metatable sets by raising an error from engine's __newindex method
2. Re-initialize engine upon script load, which seems generally like a good idea, but more architecturally invasive.

Small script to repro:

engine.name = "TestSine"

function init()
  engine.hz(math.random(10)*50+100)
end

function key(n,z)
  -- engine.hz = n * 200 -- comment/uncomment this line to test
  engine.hz(n * 200)
end

function redraw() end