Update SDL2 port to 2.0.22

[Daft-Freak]

Update to latest release, includes some threading/proxying improvements.

[Daft-Freak]

Well, that went badly: error: undefined symbol: emscripten_sync_run_in_main_runtime_thread_. From libsdl-org/SDL#5365 i think

[kripken]

That error might be an internal problem, as I don't think that function is called directly by user code. Bisecting on emscripten might be a fast way to find where things changed, if its not obvious to you already.

cc @tlively who made some recent proxying changes, and might have an idea.

[tlively]

We reimplemented emscripten_sync_run_in_main_runtime_thread_ in terms of the new Emscripten proxying API, but it is still defined in library_pthread.c, so I'm not sure why the linker would be giving you trouble about it.

And that function is called by user SDL code (via a wrapper macro) here:
https://github.com/libsdl-org/SDL/pull/5365/files#diff-edb4aebb55db5736cc47e163e4a8ff5ca56b86a24b75031c9e4d6860fd1c0756R169.

[ericoporto]

This PR will fix #16646

[Daft-Freak]

I'm guessing library_pthread.c isn't built if you're not using threads? So

if (emscripten_is_main_runtime_thread())
    do_thing();
else
    emscripten_sync_run_in_main_runtime_thread(do_thing);

maybe isn't a great idea.

(I haven't done much with threads/proxying myself, so don't really know what the right thing to do here is...)

[tlively]

Probably the simplest thing to do is hide that code behind #ifdef _REENTRANT, which is always defined if threads are enabled.

[sbc100]

Are we still trying to land this?

[Daft-Freak]

Ah, thanks for the reminder... I think I've fixed this by using MAIN_THREAD_EM_ASM for those two uses of emscripten_sync_run_in_main_runtime_thread (which everywhere else was already doing). libsdl-org/SDL#5773

This does mean we still can't use a release tag, which was part of the reason for this PR... hmm

[sbc100]

It seems reasonable to continue to use non-released revisions.. at least until we are able to.

[Daft-Freak]

Now passing after a few test adjustments (mouse events again, version number is 2.23.0 instead of 2.0.23)

[sbc100]

Maybe worth adding this to the ChangeLog?