Ask the client for frames between its steps

The frame pacer repaints the client's 3D windows at the user's frame rate
(HKCU\Software\T5DE\Graphics\FrameRate: 30, the default, adds nothing; 60, 120,
240, 360; 0 uncapped), so the client draws frames between its 30-per-second
steps. Only when affordable: a window is asked once its last frame is drawn,
and only if its frames take under three quarters of the interval; uncapped,
the client is left idle a third of a frame between frames. The engine's stats
now log frames per second.

The extra frames show the same scene for now.

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

adapter/frame_pacer.hpp

@@ -0,0 +1,24 @@
+#pragma once
+// Extra frames between the client's 30-per-second ticks. The client draws a 3D window's frame when the window gets
+// WM_PAINT, and steps its scene on its own timer (SceneWindow.dll's runApp: a step, then the window is invalidated);
+// its message loop also wakes for WM_PAINT between steps. So asking the windows to repaint more often makes the client
+// draw more frames, without stepping the scene. The rate is the user's (T5DE's settings, Graphics: HKCU\Software\T5DE\
+// Graphics, FrameRate): 30, the client's own and the default, adds nothing; 60, 120, 240 or 360 add frames between
+// ticks; 0 is uncapped.
+//
+// Drawing runs on the client's main thread, with its input and interface: extra frames are only asked for when they're
+// affordable. A window gets one only once its last frame is drawn, and only if its frames take less than three
+// quarters of the interval; uncapped, the client is left idle a third of a frame's time between frames.
+
+#include <windows.h>
+
+namespace t5de::frame_pacer {
+
+// A 3D window an engine draws into: repainted at the user's frame rate while added.
+void add(HWND window);
+void remove(HWND window);
+
+// A frame drawn into window, taking the client this long (beginRendering to endRendering).
+void frameDrawn(HWND window, double seconds);
+
+} // namespace t5de::frame_pacer