Initial Emscripten support

[felselva]

In the Emscripten backend, the function swfw_make_context probably won't do anything, but its hints can still be used in the function swfw_make_window. One of the functions will probably be the name of the canvas HTML element that will be used.

For web, there will be no support for non hardware accelerated window. The only option will be with hardware acceleration.

[felselva]

Emscripten contains functions emscripten_webgl_* that can be used to manage a WebGL context.

Emscripten documentation: https://kripken.github.io/emscripten-site/docs/api_reference/html5.h.html?highlight=emscripten_webgl_#c.emscripten_webgl_create_context