Never make the client wait for drawing, or hold up its steps

The renderer process now takes each view's chunks on a thread of its own as
they come, and draws the newest frame on its main thread: the client no longer
waits for the previous frame to be drawn (on a VM, 22 of a frame's 25 ms).

The adapter records the client's steps (SceneViewer::step, by replacing its
entry in the exported IFrameListener vtable, after checking it), and the frame
pacer doesn't ask for a frame that wouldn't end before the next step is due: a
late step made the client catch up two at once, which showed as hitches. While
the renderer process covers the view, the client's renderer doesn't begin or
present frames, which could wait for the display.

The engine's stats now include the time in the client renderer's methods.

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

adapter/scene_hooks.cpp

@@ -0,0 +1,76 @@
+#include "scene_hooks.hpp"
+
+#include <windows.h>
+
+#include <atomic>
+#include <mutex>
+
+#include "log.hpp"
+
+namespace t5de::scene_hooks {
+
+namespace {
+
+// IMVU's SceneViewer::step(), a virtual method: called with the scene viewer in ECX.
+using StepFunction = void(__thiscall*)(void* self);
+
+StepFunction originalStep = nullptr;
+std::atomic<std::int64_t> stepTime = 0;
+std::atomic<bool> hooked = false;
+
+// Our SceneViewer::step: records the tick, then steps as IMVU's does. __fastcall takes ECX like __thiscall (EDX unused).
+void __fastcall step(void* self, void* /*edx*/)
+{
+    LARGE_INTEGER now;
+    QueryPerformanceCounter(&now);
+    stepTime = now.QuadPart;
+    originalStep(self);
+}
+
+// Replaces the vtable entry at slot, if it holds expected; returns whether it did.
+bool replace(void** slot, void* expected, void* replacement)
+{
+    if (*slot != expected) {
+        return false;
+    }
+    DWORD protection = 0;
+    if (!VirtualProtect(slot, sizeof(void*), PAGE_READWRITE, &protection)) {
+        return false;
+    }
+    *slot = replacement;
+    VirtualProtect(slot, sizeof(void*), protection, &protection);
+    FlushInstructionCache(GetCurrentProcess(), slot, sizeof(void*));
+    return true;
+}
+
+} // namespace
+
+void install()
+{
+    static std::once_flag once;
+    std::call_once(once, [] {
+        HMODULE sceneWindow = GetModuleHandleW(L"SceneWindow.dll");
+        auto* vtable = sceneWindow == nullptr
+                           ? nullptr
+                           : reinterpret_cast<void**>(GetProcAddress(sceneWindow, "??_7SceneViewer@@6BIFrameListener@@@"));
+        auto* stepMethod = sceneWindow == nullptr ? nullptr : GetProcAddress(sceneWindow, "?step@SceneViewer@@UAEXXZ");
+        // IFrameListener's vtable: its destructor, then step (what stepFrame calls).
+        if (vtable == nullptr || stepMethod == nullptr) {
+            log::info("scene hooks: SceneViewer's vtable or step not found; not installed");
+            return;
+        }
+        originalStep = reinterpret_cast<StepFunction>(stepMethod);
+        if (!replace(&vtable[1], reinterpret_cast<void*>(stepMethod), reinterpret_cast<void*>(&step))) {
+            log::info("scene hooks: SceneViewer's vtable doesn't hold its step; not installed");
+            return;
+        }
+        hooked = true;
+        log::info("scene hooks: installed");
+    });
+}
+
+bool installed() { return hooked; }
+
+std::int64_t lastStep() { return stepTime; }
+
+} // namespace t5de::scene_hooks