Improve black smoke performance

[mszabo-wikia]

Changes

• Increase both minimum spread density and dissipation threshold tenfold so as not to simulate minuscule smoke clouds that all get rendered as "0 PPM" and have no appreciable impact on gameplay.
• Apply a minimum dissipation rate of 0.001 / tick so that smoke clouds < 10 PPM still dissipate in a reasonable amount of time (this should hit the 3 PPM dissipation threshold in ~3mins on speed 3).
• Use a fixed tick interval of 15 ticks for smoke.
• Double the propagation interval for smoke and also double the amount of smoke spewed by fires within this interval to compensate.
• Calculate smoke spread to adjacent cells in parallel.
• Apply black smoke-related hediffs via a postfix on GasUtility.PawnGasEffectsTickInterval() rather than a per-tick call in Smoke.

Reasoning

Our black smoke is a Thing-based gas, like all gases were prior to Biotech. This results in noticeable overhead when a large amount of smoke is present on the map, which is further exacerbated by dissipation being proportional to smoke density and thus causing small smoke clouds to take forever to disappear. Sadly there aren't really good options for modded gases these days short of creating a bespoke implementation of our own, but we can nonetheless endeavor to make the current situation better.

Alternatives

• Use an existing system that allows for modded gases - the only one I'm aware of is Simple Gas Framework, which depends on Fish and Prepatcher and therefore doesn't work in 1.6. Even if Fish gets updated someday, adding a dependency on 3 mods, where one of the authors is inactive could cause us problems the next time RimWorld updates.
• Jerry-rig a vanilla GasGrid into a MapComponent and use that for black smoke mechanics. This actually seems workable, but we'd have little control over dissipation and other functionality without ripping out parts of the code, so the end result would behave quite differently to the current black smoke.
• Implement our own bespoke gas grid for black smoke - seems like a pain to manage in the longer term.

Testing

Check tests you have performed:

• Compiles without warnings
• Game runs without errors
• Playtested a colony:

Savefile: smoke test.rws.zip

Overhead from smoke without these changes:

Overhead from smoke with these changes:

[github-actions]

You can download the rebuilt assembly for this PR here: https://combatextended.lp-programming.com/CombatExtended-20647208972.zip