Do not rely on __STDC_LIB_EXT1__ for sprintf_s

[eliemichel]

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It is not reliable at least on some versions of Clang (e.g., v14.1 on macOS 13.5.1), and anyway the safe _s functions are mostly a MSVC thing that they try and force to use through a warning but is barely standard.

Only stb_image_write was still using that, this PR aligns it to the behavior of e.g. stb_ds

[eliemichel]

Additional edit: use snprintf instead of sprintf, which is deprecated by clang, avoiding this warning:

error: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.