Graphics/Shaders: Use forward declarations where applicable

Now, with all of the headers normalized, we can safely convert some
headers into forward declarations without needing to worry about
potentially breaking code in other headers or source files.

Runtime/Graphics/Shaders/CModelShaders.hpp

@@ -1,22 +1,25 @@
 #pragma once
 
 #include <array>
-#include <optional>
+#include <cstdint>
+#include <memory>
 
 #include "DataSpec/DNAMP1/CMDLMaterials.hpp"
 
 #include "Runtime/Graphics/CGraphics.hpp"
 
-#include <hecl/Backend.hpp>
-#include <hecl/Runtime.hpp>
-
 #include <zeus/CColor.hpp>
 #include <zeus/CVector3f.hpp>
 #include <zeus/CVector4f.hpp>
 
 #define URDE_MAX_LIGHTS 8
 
+namespace hecl::Backend {
+class ShaderTag;
+}
+
 namespace urde {
+class CLight;
 
 enum EExtendedShader : uint8_t {
   Flat,