[#36][Fix] Add const keyword to return value type

include/vkcv/ShaderProgram.hpp

@@ -57,7 +57,7 @@ namespace vkcv {
 
         void reflectShader(ShaderStage shaderStage);
 
-        VertexLayout& getVertexLayout();
+        const VertexLayout &getVertexLayout() const;
 
 	private:
         std::unordered_map<ShaderStage, Shader> m_Shaders;

src/vkcv/ShaderProgram.cpp

@@ -128,7 +128,7 @@ namespace vkcv {
         m_VertexLayout = VertexLayout(inputVec);
     }
 
-    VertexLayout& ShaderProgram::getVertexLayout(){
+    const VertexLayout& ShaderProgram::getVertexLayout() const{
         return m_VertexLayout;
 	}
 }