Taper fog density at edge with a small gradient

[slipher]

Introduce the fogGradient shader keyword as suggested in #1795 (comment), which can be used to configure a gradient for fog density that tapers to 0 at the edge of the fog. The density of the fog is scaled by a factor of 1 - exp(-k * t) where t is the distance under the fog plane and k a configurable constant.

• fogGradient expFalloff <dist> configures the exponential falloff such that the density is at 50% of the maximum at <dist> qu from the fog plane.
• fogGradient const does the same thing as the current default - no gradient; hard-edged fog.

Change the default to fogGradient expFalloff 5 instead of the current constant density. Before/after comparisons:

[slipher]

Also to show what it would like with a longer gradient, here's what the Habitat example would look like with expFalloff 30.

[illwieckz]

Very nice!