[WIP] SDL2 PROXY_TO_PTHREAD Fixes

[jakogut]

This pull request fixes undefined references and proxying issues when using SDL2 with PROXY_TO_PTHREAD=1.

Requires emscripten-core/emscripten#8254

This was tested with tests/sdl2_gl_frames_swap.c and PROXY_TO_PTHREAD=1.

This still requires creating a WebGL context and making it current as documented here:
emscripten-core/emscripten#7684 (comment)

Fixes: emscripten-core/emscripten#6009

[jakogut]

@juj I'm open to feedback about a better way to do this.

[conversy]

I understood that this PR would enable the use of SDL with threads, is this correct? If so, I'm very interested into it. I'm not sure I have the skills to test it though.

[jakogut]

@conversy It enables SDL2 applications to be run with the option PROXY_TO_PTHREAD=1, which runs the main function in a separate thread.

[jakogut]

@conversy I don't know if this is what you're actually wanting, but I'm now working on SDL2 threads support through an unrelated turn of events. I've made an issue here: emscripten-core/emscripten#8300

[conversy]

My application says: "screen is not defined" when run in a browser. I understood that it was because I also use threads with SDL. Is this PR supposed to fix it?
I gave it a try (em 1.38.29, cloning SDL port, applying the PR, compiling with EMCC_LOCAL_PORTS) to no avail...

[kripken]

Curious to hear @Daft-Freak's thoughts here. My general feeling is that we can fix this another way, by replacing say Emscripten_SetWindowTitle with a new emscripten C API function emscripten_set_window_title, and we'd handle proxying of that in emscripten itself. That would be less work here, more work shared in emscripten, unless I'm missing something?

[Daft-Freak]

A lot of this could be done with new emscripten API. emscripten_set_window_title and emscripten_get_screen_size would likely be useful API additions anyway, maybe something like emscripten_set_element_style(selector, property, value) could be added for setting the cursor.

That would leave the framebuffer and creating cursors, which uses a canvas to convert some pixel data into a data-url, maybe there's some way we could do this that would also work on a worker?

[Daft-Freak]

Small comment on the PR itself: we usually just prefix commits with emscripten:

[fallenoak]

I just gave this PR a spin and it works well for my use case. What are the next steps? Implementing functions like emscripten_set_window_title in the main emscripten code base and updating this PR to make use of them?

[kripken]

@fallenoak yes, exactly, that's the right order. I don't think anyone's working on that yet, so if you or anyone else wants to, that would be great!

[jakogut]

Do we only want APIs for emscripten_set_window_title and emscripten_get_screen_size, or others as well?

[jakogut]

@kripken @Daft-Freak I've updated this PR, and started a PR in Emscripten for some of these API changes. emscripten-core/emscripten#9336