SDL2 thread proxying fixes

[NeroBurner]

Description

This PR uses new APIs added in emscripten-core/emscripten#9336 to improve compatibility with USE_PTHREADS=1.

Original PR: emscripten-ports/SDL2#127
By: @jakogut
Reviewed by: @Daft-Freak

This fix enables me to compile an executable using -s USE_PTHREADS=1 -s USE_SDL=2 and not get the following error in the chromuim debug console

Uncought TypeError: Cannot read properties of undefined (reading 'CreateImageData')

With the current patch applied the InfiniSim Simulator works 🎉

Existing Issue(s)

• SDL2 application will break when using pthreads emscripten-core/emscripten#6009

[Daft-Freak]

I think some of the comments from the original PR still apply, though I still haven't really looked into emscripten's threading stuff...

[Daft-Freak]

Minor cast warning and formatting comments. Also SDL_GetDisplayUsableBounds has some more EM_ASMs that were added after the original PR.

[NeroBurner]

I hope I addressed the required changes. Can't follow about SDL_GetDisplayUsableBounds as I don't think I've touched that in this PR. Is there something I should update anyways? Just trying to "champion" these changes, as they solve an obstacle for me.

[uyjulian]

Yeah you should update SDL_GetDisplayUsableBounds to also proxy to main thread

[icculus]

@Daft-Freak Do we want to merge this and deal with SDL_GetDisplayUsableBounds separately, or should one of us add it to this PR?

[Daft-Freak]

It's probably just EM_ASM_INT_V -> MAIN_THREAD_EM_ASM_INT there too, I haven't got around to trying it though. Okay with merging as-is or with the extra change if someone tests it.

[icculus]

I added the SDL_GetDisplayDisplayBounds stuff in ea7d530, so this is good to go now.

[NeroBurner]

thanks a lot @icculus 🙇‍♂️