Resolved multiple -Wsign-compare warnings

[SAN4EZDREAMS]

Fixes numerous signed/unsigned comparison warnings reported by recent GCC versions across the code.
Changes involve explicit type casting to ensure safe comparisons without altering the original logic.

Exception (third-party library):

/home/runner/work/gzdoom/gzdoom/src/common/thirdparty/stb/stb_sprintf.h:630:51: warning: operand of ‘?:’ changes signedness from ‘int’ to ‘unsigned int’ due to unsignedness of other operand [-Wsign-compare]
  630 |          l = stbsp__strlen_limited(s, (pr >= 0) ? pr : ~0u);
      |                                                   ^~

[RicardoLuis0]

the fix to this isn't casting everywhere, that just hides the warnings, it's to change indices/etc to the proper sign where possible