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/eframe/src/stopwatch.rs

@@ -0,0 +1,50 @@
+#![allow(dead_code)] // not everything is used on wasm
+
+use web_time::Instant;
+
+pub struct Stopwatch {
+    total_time_ns: u128,
+
+    /// None = not running
+    start: Option<Instant>,
+}
+
+impl Stopwatch {
+    pub fn new() -> Self {
+        Self {
+            total_time_ns: 0,
+            start: None,
+        }
+    }
+
+    pub fn start(&mut self) {
+        assert!(self.start.is_none());
+        self.start = Some(Instant::now());
+    }
+
+    pub fn pause(&mut self) {
+        let start = self.start.take().unwrap();
+        let duration = start.elapsed();
+        self.total_time_ns += duration.as_nanos();
+    }
+
+    pub fn resume(&mut self) {
+        assert!(self.start.is_none());
+        self.start = Some(Instant::now());
+    }
+
+    pub fn total_time_ns(&self) -> u128 {
+        if let Some(start) = self.start {
+            // Running
+            let duration = start.elapsed();
+            self.total_time_ns + duration.as_nanos()
+        } else {
+            // Paused
+            self.total_time_ns
+        }
+    }
+
+    pub fn total_time_sec(&self) -> f32 {
+        self.total_time_ns() as f32 * 1e-9
+    }
+}