Add stb_image.

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doc/preview_surface.md

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+# PreviewSurface
+
+https://graphics.pixar.com/usd/docs/UsdPreviewSurface-Proposal.html 
+
+## Inputs
+
+* diffuseColor - color3f - (0.18, 0.18, 0.18)
+* emissiveColor - color3f - (0.0, 0.0, 0.0)
+* useSpeclarWorkflow - int - 0
+  * 1
+    * specularColor - color3f - (0.0, 0.0, 0.0)
+  * 0
+    * metallic - float - 0.0
+* roughness - float - 0.5
+* clearcoat - float - 0.0
+* clearcoatRoughness - float 0.01
+* opacity - float - 1.0
+* opacityThreshold - float - 0.0
+* ior - float - 1.5
+* normal - normal3f - (0.0, 0.0, 1.0)
+* displacement - float - 0.0
+* occlusion - float - 1.0
+
+## Outputs
+
+* surface - token
+* displacement - token
+
+## UsdUVTexture
+
+### Inputs
+
+* file - asset - string
+* st - float2 - (0.0, 0.0)
+* wrapS - token - useMetadata
+  * black, clamp, repeat, mirror, 
+* wrapT - token = useMetadata
+* fallback - float4 - (0.0, 0.0, 0.0, 1.0)
+* scale - float4 - (1.0, 1.0, 1.0, 1.0)
+* bias - float4 - (0.0, 0.0, 0.0, 0.0)
+
+### Outputs
+
+* r, g, b, a
+
+
+## UsdTransform2d
+
+### Inputs
+
+* in - float2 - (0.0, 0.0)
+* rotation - float - (0.0)
+  * counter-clockwise rotation in degrees around the origin
+* scale - float2 - (1.0, 1.0)
+* translation - float2 (0.0, 0.0)
+
+### Outpuits
+
+* result - float2
+
+
+## TODO
+
+* [ ] Primvar Reader(arbitrary vertex attributes)
+* [ ] displacement
+* [ ] Texture transform
+