Add the renderer process

The renderer becomes two programs: the adapter (t5deRenderEngine.dll, 32-bit, in IMVU's process) and a 64-bit renderer process (t5de-renderer.exe) drawing with Diligent Engine, fetched with FetchContent and pinned. For now a spike: the adapter starts the renderer process for each engine and signals each frame through a shared memory channel, and the renderer process draws a box into a child window in the client's 3D view.

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

include/t5de/imvu/types.hpp

@@ -5,6 +5,8 @@
 #include <cstddef>
 #include <cstdint>
 
+struct HWND__; // windows.h's window handle type (HWND is HWND__*)
+
 namespace t5de::imvu {
 
 template <class T>
@@ -94,6 +96,18 @@ struct PointSpriteVector;
 // Region lists IMVU passes to texture updates; unknown so far.
 class CompoundRegion;
 
-class INativeWindow;
+struct UiEvent;
+using NativeWindowHandle = ::HWND__*; // an HWND
+
+// IMVU's window (SceneWindow.dll's NativeWindow), as far as a renderer needs it: the first methods of its vtable.
+// Its later methods include overloads, which MSVC lays out unlike their declaration: declare them from the vtable
+// if they're ever needed.
+class INativeWindow {
+public:
+    virtual ~INativeWindow() = default;
+    virtual void handleMessage(UiEvent* event) = 0;
+    virtual int onSetCursor(Vec2i position) = 0;
+    virtual NativeWindowHandle getPlatformWindow() = 0;
+};
 
 } // namespace t5de::imvu