Add a pass-through renderer plugin

t5deRenderEngine.dll implements IMVU Classic's renderer plugin interface by wrapping IMVU's own OpenGL renderer, and logs what each frame calls, to prove our declarations of the interface. Built with CMake presets for 32-bit MSVC; docs/interface.md has the findings.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/plugin.def

@@ -0,0 +1,5 @@
+; The names SceneWindow.dll's createDLLRenderer looks up, without C++ or __cdecl decoration.
+LIBRARY t5deRenderEngine
+EXPORTS
+    isRenderEngineSupported
+    createRenderEngine