Improve MeshPhysicalMaterial transmission support (mrdoob#21884)

* Imptove MeshPhysicalMaterial transmission support

* Add webgl_loader_gltf_transmission example

* Update webgl_materials_physical_transmission screenshot

* Dispose transmissionRenderTarget in WebGLRenderer.dispose()

* Remove _transmissionSamplerMap from MeshPhysicalMaterial

* Fix transmissionMap

* Remove unnecessary sRGB-Linear conversion from transmission shader

* Improve transmission shaders

* Add more parameters to webgl_materials_physical_transmission example

* Update screenshot

examples/files.json

@@ -86,6 +86,7 @@
 		"webgl_loader_gltf",
 		"webgl_loader_gltf_compressed",
 		"webgl_loader_gltf_extensions",
+		"webgl_loader_gltf_transmission",
 		"webgl_loader_gltf_variants",
 		"webgl_loader_ifc",
 		"webgl_loader_imagebitmap",

examples/models/gltf/TransmissionTest/README.md

@@ -0,0 +1,14 @@
+# Transmission Test
+
+## Screenshot
+
+![screenshot](screenshot/screenshot_large.png)
+
+## Description
+
+This model demonstrates the usage of the `KHR_materials_transmission`. The grid of spheres demonstrate different combinations of uniform transmission and textured transmission and how they should render in conjunction with opacity (`baseColor.a`) and surface roughness/metalicity.
+
+## License Information
+
+[![CC0](http://i.creativecommons.org/p/zero/1.0/88x31.png)](http://creativecommons.org/publicdomain/zero/1.0/)  
+To the extent possible under law, Adobe has waived all copyright and related or neighboring rights to this asset.