[client] overlay: initialize ImGui frame timing

Initialize the previous ImGui frame timestamp before rendering
overlays. Without this, the first DeltaTime equals host uptime.

Recent ImGui versions track background and foreground draw-list
activity using float time. At long host uptimes its precision can span
multiple display frames, preventing those lists from being reset and
submitting translucent overlay commands more than once.

client/src/app.c

@@ -962,6 +962,10 @@ void app_initOverlays(void)
     }
   }
   ll_unlock(g_state.overlays);
+
+  /* Do not seed ImGui's clock with absolute host uptime. ImGui stores the
+   * background and foreground draw-list activity time as a float. */
+  g_state.lastImGuiFrame = nanotime();
 }
 
 static inline void mergeRect(struct Rect * dest, const struct Rect * a, const struct Rect * b)