Debloat IVideoDriver and IrrlichtDevice includes (#15080)

As the project grows, compile time will not go down unless the header mess
is cleaned up one by one to only include exactly what's needed.

src/client/render/core.h

@@ -21,6 +21,11 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #pragma once
 #include "irrlichttypes_extrabloated.h"
 
+namespace irr
+{
+	class IrrlichtDevice;
+}
+
 class ShadowRenderer;
 class Camera;
 class Client;