Fix Emscripten CI tests

[fancycode]

#925 broke the Emscripten CI, so adding an issue to bring back testing of Emscripten.

Originally posted by @fancycode in #925 (comment)

[farindk]

Yes, I tried to fix the emscripten thing a couple of days ago, but I failed to understand what is going on in the code. It would be nice if you could have a look.

[jerbob92]

I believe the JS test might not be correct. The functions are not directly available after the import (afaik), so in this code:

    var libheif = require('../libheif.js');
    console.log("Loaded libheif.js", libheif.heif_get_version());

I don't think heif_get_version is defined directly afterwards. It could be that was the case with an older version of Emscripten.

This is also why https://github.com/strukturag/libheif/blob/master/examples/demo.html runs on window.onload.

Another issue is that it cuts off the logging, so its hard to see what is actually wrong.

[fancycode]

So there is a regression which will break existing users code after an update. The old version of the JS code published a global libheif (used by the demo) or exported it if used from require (used in the CI tests).

In any case, that's what this issue is for ;-) The broken behaviour should be fixed.

[jerbob92]

In that case, probably. I only looked at the example.html, and that still worked, didn't know there was a unit test.

Not completely sure if it can be fixed in Emscripten, I think they intentionally changed it to work this way.

Perhaps we should look at how it currently wraps the generated JS too, it does some magic which shouldn't really be necessary anyway: https://github.com/strukturag/libheif/blob/master/pre.js#L35 and https://github.com/strukturag/libheif/blob/master/post.js

[jerbob92]

Reading this page it seems that WASM_ASYNC_COMPILATION=0 would fix it, but it comes with some sidenotes: but that may not work in Chrome due to current limitations there..

Maybe we can add the option when USE_WASM is 0, because in that case it doesn't have to load WebAssembly and that should make it loadable in a sync way, USE_WASM=0 is also what the test uses.