Merge pull request #167 from KhronosGroup/DiffuseTransmissionTest

Merging this unit test asset.

Models/DiffuseTransmissionTest/README.body.md

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+## Screenshot
+
+![screenshot](screenshot/screenshot_Large.jpg)<br/>
+_Screenshot from the [glTF Sample Viewer](https://github.khronos.org/glTF-Sample-Viewer-Release/) with the environment Cannon Exterior._
+
+## Description
+
+This model tests diffuse transmission inputs for [`KHR_materials_diffuse_transmission`](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#readme). 
+
+There is a single directional punctual light, facing the camera. 
+
+The smaller rectangles on the left side use Unlit materials to indicate color and texture inputs.
+
+Behind each of the larger rectangles are dark pink rectangles, to emulate the layout as seen [in the extension readme here](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#diffusetransmissionfactor). However these may only affect the diffuse transmission in renderers using pathtracing.
+
+![render from Dassault Stellar](screenshot/dassault-stellar.jpg)<br/>
+_Path-traced render from Dassault Systèmes [Stellar Physically Correct](https://blog.3ds.com/brands/3dexcite/stellar-physically-correct/), using the PBR Neutral tonemapper._
+