update color at picking and cleanup code from some warnings

src/util.h

@@ -87,7 +87,7 @@ inline glm::ivec3 xyz(const glm::ivec4& v) { return glm::ivec3(v.x, v.y, v.z); }
 inline glm::ivec2 zw(const glm::ivec4& v) { return glm::ivec2(v.z, v.w); }
 inline glm::vec2 xy(const glm::vec3& v) { return glm::vec2(v.x, v.y); }
 
-void parallel_for(unsigned nb_elements, std::function<void(int i)> functor, bool use_threads = true);
+void parallel_for(size_t nb_elements, std::function<void(size_t i)> functor, bool use_threads = true);
 
 template<typename T> struct cbuffer
 {