change GetColor parameter to size_t

#470

library/phx/resources/types/lookup_table.cpp

@@ -40,7 +40,7 @@ void LookupTable::Generate(int num_colors) {
         transfer_function_.Interpolate(min_key_ + i * step));
   }
 }
-glm::vec4 LookupTable::GetColor(int index) const {
+glm::vec4 LookupTable::GetColor(std::size_t index) const {
   return lookup_table_.at(index);
 }
 

library/phx/resources/types/lookup_table.hpp

@@ -45,7 +45,7 @@ class PHOENIX_EXPORT LookupTable {
   ~LookupTable() = default;
 
   std::size_t GetIndex(float key) const;
-  glm::vec4 GetColor(int index) const;
+  glm::vec4 GetColor(std::size_t index) const;
   std::size_t GetSize() const;
 
   std::unique_ptr<gl::texture_1d> GetTexture1D() const;