Add dragon attenuation model.

[emackey]

See the README.

[emackey]

Things to consider here:

• Scale the thicknessFactor from 3.5 to 2.2, and scale attenuationDistance by the same percentage. This would get the thicknessFactor to more closely match the raw vertex distances at the thickest portion of the dragon's body.
• Change the thickness texturemap here to only use the green channel, to make sure implementations are reading from the correct channel. The AttenuationTest model handles this test now.

[emackey]

Actually I'm not sure 2.2 is the thickest part, it looks like some lower parts are above 3.0. Would be interesting to compare to the path tracer if they add support for attenuation there.

[emackey]

Also:

• Add "common problems" section with a screenshot of what happens when the node scaling is not accounted for.

[emackey]

This is ready for review.

[abwood]

Great model and fantastic writeup. Thanks @emackey!