Add: support for emscripten (play-OpenTTD-in-the-browser)

Emscripten compiles to WASM, which can be loaded via
HTML / JavaScript. This allows you to play OpenTTD inside a
browser.

Co-authored-by: milek7 <me@milek7.pl>

os/emscripten/README.md

@@ -0,0 +1,40 @@
+## How to build with Emscripten
+
+Building with Emscripten works with emsdk 2.0.10 and above.
+
+Currently there is no LibLZMA support upstream; for this we suggest to apply
+the provided patch in this folder to your emsdk installation.
+
+For convenience, a Dockerfile is supplied that does this patches for you
+against upstream emsdk docker. Best way to use it:
+
+Build the docker image:
+```
+  docker build -t emsdk-lzma .
+```
+
+Build the host tools first:
+```
+  mkdir build-host
+  docker run -it --rm -v $(pwd):$(pwd) -u $(id -u):$(id -g) --workdir $(pwd)/build-host emsdk-lzma cmake .. -DOPTION_TOOLS_ONLY=ON
+  docker run -it --rm -v $(pwd):$(pwd) -u $(id -u):$(id -g) --workdir $(pwd)/build-host emsdk-lzma make -j5 tools
+```
+
+Next, build the game with emscripten:
+
+```
+  mkdir build
+  docker run -it --rm -v $(pwd):$(pwd) -u $(id -u):$(id -g) --workdir $(pwd)/build emsdk-lzma emcmake cmake .. -DHOST_BINARY_DIR=$(pwd)/build-host -DCMAKE_BUILD_TYPE=RelWithDebInfo -DOPTION_USE_ASSERTS=OFF
+  docker run -it --rm -v $(pwd):$(pwd) -u $(id -u):$(id -g) --workdir $(pwd)/build emsdk-lzma emmake make -j5
+```
+
+And now you have in your build folder files like "openttd.html".
+
+To run it locally, you would have to start a local webserver, like:
+
+```
+  cd build
+  python3 -m http.server
+````
+
+Now you can play the game via http://127.0.0.1:8000/openttd.html .