Make unused variable removal safer

[laurentlb]

The behavior is documented (https://github.com/laurentlb/Shader_Minifier#unused-local-variables), but it can be risky to remove the initialization code of a variable that is not used.

Code example:

out vec4 outcolor;
float glow = 0.;

vec2 map (in vec3 p) {
  glow = 1.;
  // ...
  return vec2(1);
}

vec2 march(vec3 ro, vec3 rd) {
  vec2 t = map(p);
  return vec2(1);
}

void main()
{    
    vec3 rd = vec3(1);
    vec3 ro = vec3(0);
    vec2 t = march(ro,rd);        
    //vec3 col = t.y*0 + abs(vec3(glow)*.65);        
    vec3 col = abs(vec3(glow)*.65);        
    outcolor = vec4(col,1.0);            
}

The user commented a use of the variable t, which deleted the call to march although it has a side-effect and modifies glow. We'd need a proper way to track side-effects.