Add minimal emscripten support

[zturtleman]

This is a minimal emscripten port that will build and run. It supports the OpenGL2 renderer using WebGL 1.0 and 2.0. It uses the generated html file from emscripten and "--preload-file" to merge all pk3s into a single file that emscripten loads as a VFS.

There is a lot of room to improve different aspects like pk3 loading, html integration, loading game logic from .wasm, and build flags. (Which is to say this is not necessarily better or more complete than the several other ports using emscripten.)

1. Create "baseq3" directory in the same directory as the Makefile.
2. Copy pak[0-8].pk3 into the created "baseq3" directory.
3. Run source "/path/to/emsdk_env.sh" to add emcc to PATH.
4. Run make PLATFORM=emscripten
5. Serve the build/release-emscripten-wasm32/ioquake3_opengl2.{html,js,wasm,data} from a web server. (Local files aren't allowed to load wasm.)
6. Load ioquake3_opengl2.html in a web browser from the web server.