Much more accurate cpu_usage timing (#3913)

`frame.info.cpu_usage` now includes time for tessellation and rendering,
but excludes vsync and context switching.

crates/egui_demo_app/src/frame_history.rs

@@ -38,8 +38,8 @@ impl FrameHistory {
             1e3 * self.mean_frame_time()
         ))
         .on_hover_text(
-            "Includes egui layout and tessellation time.\n\
-            Does not include GPU usage, nor overhead for sending data to GPU.",
+            "Includes all app logic, egui layout, tessellation, and rendering.\n\
+            Does not include waiting for vsync.",
         );
         egui::warn_if_debug_build(ui);