flood_water would occasionally corrupt pmap by moving the pmap entry at i's location even if the entry itself didn't refer to i. pmap updates are tricky, they are best handled by Simulation::move.
Saving blockair/blockairh is nice because RecalculateBlockAirMaps uses the sim rng, which means the sim rng would get advanced in Simulation::Load. Also rename RecalculateBlockAirMaps to ApproximateBlockAirMaps because that's what it is, an approximation, and it's needed only if there are no block air maps in the save.
Simulation::frameCount keeps track of frames elapsed since the beginning of the simulation, zeroed at clear_sim. It overflows when it reaches the 64-bit limit, which means anything that depends on it should either handle this, or not fail catastrophically. sandcolour (the only thing that depends on it as of now) is a good example of the latter: sandcolour has a periodicity of 360 frames, which means that there is one sandcolour period that is cut short by the overflow. This is not "handled" (the period is cut short, which is detectable by users) but is not catastrophic either (it's not a big deal, and it won't ever happen unless someone hacks the save).
Also restrict saves with determinism data to 98.0.
Achieved by adding a new element property called CarriesTypeIn, whose bits signal to save loading code which properties of particles of the element class in question carry element IDs. The bits in this property are numbered the same way as sim.FIELD_* constants for consistency. One would signal from Lua that a custom element carries element IDs in its tmp like this:
elem.property(id, "CarriesTypeIn", 2 ^ sim.FIELD_TMP)
"Carrying an element ID in a property" is to be interpreted as follows: the property is treated as a combination of a PMAPBITS-bit (so, currently 9-bit) unsigned integer lower part holding an element ID and a 32-PMAPBITS-bit (so, currently 23-bit) signed integer upper part holding whatever makes sense for the element. CONV, for example, uses this signed integer in its ctype as the extra "v" parameter for particle creation.
Some checks on particles, most importantly whether their element IDs refers to an enabled element, were done _before_ in-save element IDs are mapped to in-simulation element IDs. This resulted in some particles being removed if their IDs were unlucky enough.
A bug existed before where certain events would not update Engine's lastTick. If the sim was lagging hard, then this could cause "script is not responding" errors to appear in unintentional situations.
The starting execution time is tracked in LuaScriptInterface instead now, and set in tpt_lua_pcall
Also replace a few rename calls with RenameFile calls. Old code doesn't expect rename to overwrite existing files without question, when it in fact can.
These are the only bit of shared state between the Request user thread and RequestManager that aren't covered by RequestHandle::stateMx. The problem was that they were not covered by anything, which meant that they were not guaranteed to be coherent between threads.
Also fix WriteFile being unable to overwrite existing files. The rename would fail because the file was still open, and the sanity remove in response to that would also fail for the same reason.
