fix(glsl-lang-pp): eval and expand #elif only if the last is none

[yorkie]

This pull request refines the handling of conditional preprocessing directives in the macro expansion logic, specifically improving how elif directives are evaluated based on the current state of the if stack.

For example:

#version 300 es
#define CS1
#define CS2
#define CS3

vec3 test() {
#if defined(CS1)
  return vec3(1.0, 0.0, 0.0);
#elif defined(CS2)
  return vec3(2.0, 0.0, 0.0);
#elif defined(CS3)
  return vec3(3.0, 1.0, 0.0);
#else
  return vec3(0.0, 0.0, 1.0);
#endif
}

Without this patch, it will be expanded to:

#version 300 es
vec3 test() {
  return vec3(1.0, 0.0, 0.0);
  return vec3(2.0, 0.0, 0.0);
  return vec3(3.0, 1.0, 0.0);
#endif
}

[yorkie]

/cc @alixinne

[alixinne]

Sorry about the delay, I was away from a keyboard these past few weeks and just came back. I'll look into it this week 👍

[alixinne]

Thanks a lot for the fix 🚀