Add ktx

ktx/external/basisu/webgl/transcoder/README.md

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+Prebuilt versions of `basis_transcoder.js` and `basis_transcoder.wasm` are included in the `build/` folder, and are sufficient for local demos. To build the transcoder yourself, first install emscripten ([tutorial](https://webassembly.org/getting-started/developers-guide/)) and cmake ([download](https://cmake.org/download/)). Then run:
+
+```shell
+cd webgl/transcoder/build/
+emcmake cmake ../
+make
+```