Handle WebGL context loss about wasm4

[takaeri]

Once #44 lands, I'll work on handling this issue

[aduros]

Thanks, yeah, it looks like it should be pretty easy. The GL init stuff in compositor.js needs to be pulled out into a function, and that function called on webglcontextrestored. composite() should also return early if gl.isContextLost().

Let's get #44 merged first, since it touches a lot of the same code.

A PR would be very welcome!

[takaeri]

Take a look into this

1. add a lost context handler and tell it to prevent the default behavior

var canvas = document.getElementById("myCanvas");
canvas.addEventListener("webglcontextlost", function(event) {
    event.preventDefault();
}, false);

2. re-setup all your WebGL state and re-create all your WebGL resources when the context is restored.

canvas.addEventListener(
    "webglcontextrestored", setupWebGLStateAndResources, false);

I can submit a PR but I'm quite busy this week

[aduros]

#44 has landed if you still want to take a crack at this one 😃