robust puddle viscosity (again)

[Darkmajia]

puddles now slow by the average viscosity of the solution, rather than only applying the lowest and highest viscosity values. this means you can now dilute puddles to make them slow down less. small puddles also now only apply half the viscosity adjustment

robust.puddles.webm

Changelog
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• add: Puddles can now be diluted to apply less slow-down and speed-up.
• tweak: Small puddles only apply half the speed adjustment of large ones.