The new `linearFog` material property, when set to `true` enables a
simplified fog calculation. The fog equation becomes linear which is
unrealistic, but more efficient to compute. In some situations with
a shallow fog range, it doesn't make a huge difference visually.
In this mode, height falloff and in-scattering are ignored.
The linear equation slope is calculated from the regular parameters to
match the slope of the real equation at a camera height. If
`heightFalloff` is disabled, set to 0, the `density` parameter
exactly corresponds to the slope of the equation in [1/m] units.
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