We were using `Content-Length` from the server when `Content-Encoding`
was not set (i.e. response was not compressed).
Sadly, in CORS requests accessing headers is restricted, and while
`Content-Length` is enabled by default, `Content-Encoding` is not.
This results in the impossibility of knowing if the content was
compressed, unless the server explicitly enabled the encoding header
via `Access-Control-Expose-Headers`.
To keep maximum compatibility we must disable `body_size` completely.
This allow the loading bar to be much more reliable, even in cases where
realible stream loading status is not detectable (server-side
compression, chunked encoding).
Initial work to make liniting easier.
This includes:
- Rename http_request.js to library_godot_http_request.js.
- Rename externs.js to engine.externs.js.
- New library_godot_runtime.js (GodotRuntime) wraps around emscripten
functions.
- Refactor of XMLHttpRequest handler in engine/preloader.js.
- Few fixes to bugs spotted by early stage linting.
