vertex_loader: Provide an assertion for ensuring the loader has been setup

Also adds an assert to ensure that Setup is not called more than once
during a VertexLoader's lifetime.

src/video_core/vertex_loader.h

@@ -34,6 +34,7 @@ private:
     std::array<u32, 16> vertex_attribute_elements{};
     std::array<bool, 16> vertex_attribute_is_default;
     int num_total_attributes = 0;
+    bool is_setup = false;
 };
 
 }  // namespace Pica