Added new hint: SDL_HINT_EMSCRIPTEN_CANVAS_ELEMENT

[sherpya]

This helps emscripten to find the canvas HTML element

Description

This patch add the possibility to specify the canvas using a new hint SDL_HINT_EMSCRIPTEN_CANVAS_ELEMENT

Existing Issue(s)

fixes #5260

Non related, a strange side effect while testing the patch, calling SDL_CreateWindow with an overside dimension takes the path (I've added an fprintf before):

    /* Some platforms blow up if the windows are too large. Raise it later? */
    if ((w > 16384) || (h > 16384)) {
        SDL_SetError("Window is too large.");
        return NULL;
    }

but SDL_GetError() returns SDL not built with thread support, that is the output of SDL_CreateSemaphore() in src/thread/generic/SDL_syssem.c, I don't known who calls it, perhaps I may make a PR that prefixes the name of the function called in that file

The only possible caller looks like SDL_TimerInit(), but

#if !defined(__EMSCRIPTEN__) || !SDL_THREADS_DISABLED

that shouldn't be compiled in

[Daft-Freak]

I have almost the same patch: Daft-Freak@b0ed9e0. Though I went with ..._CANVAS_SELECTOR (not that it matters much).

It should probably be mentioned that everything rendering related is going to ignore this and use Module.canvas without something like Daft-Freak@cb6d810 / Daft-Freak@d3d0169.

(I was about to PR the first four commits from Daft-Freak/SDL@main...Daft-Freak:SDL:create-window-from-2 after being reminded about #5260.)

[Daft-Freak]

Should be canvas_element (or even canvas_selector), also there's an extra semicolon.

[sherpya]

ok for the semicolon, I've updated the PR, but I've used canvasElement according to:
https://github.com/libsdl-org/SDL/blob/main/src/video/emscripten/SDL_emscriptenevents.c#L722

and anyway all the code calls it element, e.g. emscripten_set_canvas_element_size
The variable name is not really a problem for me, I can change it

[Daft-Freak]

Yeah, that one is also wrong... most names are in snake_case now. (You could argue that that one is even more wrong as some of the options aren't even elements... But uh, it is also 8 year old code 😄 )

... and yeah there's a bit of inconsistent naming going on, mostly because it used to be an element id(+ some magic values) and is now a CSS selector (+ some other magic values).

[sherpya]

so? @slouken @Daft-Freak going for canvas_element or canvas_selector?

[sherpya]

and yes I handle directly specialHTMLTargets object because my canvas is in a shadow dom, emscripten wouldn't be able to find it without the shadow dom root

[slouken]

I'm going to close this for now in favor of @Daft-Freak's work in progress.