[sprintf] Support for 16-bit int systems

[stefano-zanotti-88]

The library makes various assumptions about the size of primitive types.
In particular, it assumes that sizeof(int) == 4.
It would be nice to be more standard and respect the actual int size of the system, as well as have better checks for the proper size to use for the 'j' 'z' 't'.
'z' now checks for the wrong type -- prdiff_t rather than size_t; and 'j' checks for 'size_t' rather than 'intmax_t', in order to avoid including more standard headers:

stb/stb_sprintf.h

Lines 543 to 555 in f056911

	case 'j':
	fl |= (sizeof(size_t) == 8) ? STBSP__INTMAX : 0;
	++f;
	break;
	// are we 64-bit on size_t or ptrdiff_t? (c99)
	case 'z':
	fl |= (sizeof(ptrdiff_t) == 8) ? STBSP__INTMAX : 0;
	++f;
	break;
	case 't':
	fl |= (sizeof(ptrdiff_t) == 8) ? STBSP__INTMAX : 0;
	++f;
	break;