JPEG: Handle errors when saving with libjpeg

[smcv]

• JPEG: Factor out the middle of IMG_SaveJPG_RW_jpeglib

  No functional change intended. We'll need to use setjmp() in this function in a subsequent commit, so ensure that its state doesn't include any local variables that are used both "above" and "below" the stack level at which we will call setjmp().

• JPEG: Add error-recovery when saving with libjpeg

  Because we have set up libjpeg to use my_error_exit, we need to call setjmp() before the first time it might possibly call longjmp(). Otherwise, on error we'll do a non-local goto to an uninitialized pointer and crash.

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SDL3 version of #432. Untested (there's no SDL3 in the scout SDK yet) but the only difference is the name of a constant.

[smcv]

The Windows CI build says:

D:/a/SDL_image/SDL_image/src/IMG_jpg.c:489:18: error: variable 'jpeg_surface' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]

[sezero]

Maybe we should separate longjmp handling and the rest like we did for libpng code in https://github.com/libsdl-org/SDL_image/commit/3e42a4fb0138f1f2522c43a9e1cda41d24c2a21c (followed by minor simplification https://github.com/libsdl-org/SDL_image/commit/135bec7c30a00703b3c1a6aab04743ab522262fe) (also applied to SDL2 branch)

[smcv]

Maybe we should separate longjmp handling and the rest like we did for libpng code in 3e42a4f (followed by minor simplification 135bec7) (also applied to SDL2 branch)

Yes, I think that's going to be necessary. I'm trying to work out how to do that in the most reviewable way.

[smcv]

OK, how's this?

[sezero]

Looks good, I think. We should do the same to the load function too.

[smcv]

We should do the same to the load function too.

Ideally yes, but I'd prefer that sort of refactoring not to be a blocker for a crash fix.

[sezero]

@slouken ?

[slouken]

Looks good!

[sezero]

We should adapt to SDL2 too and do the same for loader function as well, later.

[smcv]

We should adapt to SDL2 too

That was #432 (which is actually the version I wrote first)

and do the same for loader function as well, later

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