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

include/t5de/protocol/channel.hpp

@@ -0,0 +1,25 @@
+#pragma once
+// What the adapter (t5deRenderEngine.dll, in IMVU's 32-bit process) and the renderer process (64-bit) share. Both
+// are built from this header, for different architectures: fixed-size fields only, no pointers or handles' native
+// sizes.
+
+#include <cstdint>
+
+namespace t5de::protocol {
+
+inline constexpr std::uint32_t version = 1;
+
+// A shared memory section the adapter creates for each engine. Its name is passed to the renderer process as
+// `--channel <name>`; the event "<name>-frame" (auto-reset) is set after each frame the client draws.
+struct Channel {
+    std::uint32_t version;
+    std::uint32_t closing; // nonzero: the engine is gone, the renderer process should exit
+    std::uint64_t window;  // IMVU's 3D window (an HWND; window handles are valid in every process)
+    std::uint64_t frames;  // frames the client has drawn
+};
+
+static_assert(sizeof(Channel) == 24, "same layout in 32-bit and 64-bit builds");
+
+inline constexpr wchar_t frameEventSuffix[] = L"-frame";
+
+} // namespace t5de::protocol