Emscripten

[konsumer]

This is related to #36 but uses plain emscripten (no need for SDL-shim.)

There are a couple of caveats.

• Because of how the main-loop and sleep works in fenster, you need ASYNCIFY, which adds a lot to js. This could be avoided, but would require changes to how fenster works (have a wrapper for a function like fenster_set_loop(cb) and don't use fenster_loop or fenster_sleep.) Asyncify also adds performance overhead, so getting rid of that might really be worth exploring.
• I keep it in a seperate header, because I like the idea of conditionally including it (with #if EMSCRIPTEN) so people can choose if they want emscritpen support easily, sort of like audio.

I updated these examples, including Makefile:

• drawing-c
• input-c
• minimal-c

For each, they will build for native with make, or for web with emmake make

some things that could be improved later:

• no sound, but it could be added
• I don't like adding event-listeners without a way to remove them. it's in js-space, so I think they need to be inserted into Module so they can be removed in fenster_close
• keys need to be very closely checked. All my keyboard-keys worked on linux/chrome and mac/chrome, but I don't think there is any guarantee these keycodes will match, across browsers or platforms
• fenster-doom would be a nice "kitchen sink" demo for web
• didn't really attempt to port any other languages (go, lua, etc) to emscripten-method. They have their own ways of binding to web-wasm, and it can get complicated.