Further shader compilation development.

include/opengl/handles/shader_handle.hpp

@@ -8,6 +8,11 @@ struct shader_handle
 {
 	constexpr bool operator==(const shader_handle&) const = default;
 
-	GLuint shader_id{ 0 };
+	constexpr bool valid() const
+	{
+		return id != 0;
+	}
+
+	GLuint id{ 0 };
 };
 }