web: Use initial timestamp from requestAnimationFrame

Previously Performance.now() was used to grab the initial timestamp
for calculating dt in requestAnimationFrame. However, this doesn't
seem to be reliable and resulted in negative dt values in Chrome.

Now we just use an Option for the timestamp and initialize it to None.
The first animation callback will set the timestamp.

web/src/lib.rs

@@ -25,7 +25,7 @@ struct RuffleInstance {
     canvas_width: i32,
     canvas_height: i32,
     device_pixel_ratio: f64,
-    timestamp: f64,
+    timestamp: Option<f64>,
     animation_handler: Option<AnimationHandler>, // requestAnimationFrame callback
     animation_handler_id: Option<NonZeroI32>,    // requestAnimationFrame id
     #[allow(dead_code)]
@@ -84,11 +84,6 @@ impl Ruffle {
 
         let core = ruffle_core::Player::new(renderer, audio, data)?;
 
-        let timestamp = window
-            .performance()
-            .ok_or_else(|| "Expected performance")?
-            .now();
-
         // Create instance.
         let instance = RuffleInstance {
             core,
@@ -102,7 +97,7 @@ impl Ruffle {
             mouse_move_callback: None,
             mouse_down_callback: None,
             mouse_up_callback: None,
-            timestamp,
+            timestamp: None,
         };
 
         // Register the instance and create the animation frame closure.
@@ -226,8 +221,8 @@ impl Ruffle {
             ruffle
         });
 
-        // Do an initial tick to start the animation loop.
-        ruffle.tick(timestamp);
+        // Set initial timestamp and do initial tick to start animation loop.
+        ruffle.tick(0.0);
 
         Ok(ruffle)
     }
@@ -236,8 +231,18 @@ impl Ruffle {
         INSTANCES.with(|instances| {
             let mut instances = instances.borrow_mut();
             if let Some(instance) = instances.get_mut(self.0) {
-                let dt = timestamp - instance.timestamp;
-                instance.timestamp = timestamp;
+                // Calculate the dt from last tick.
+                let dt = if let Some(prev_timestamp) = instance.timestamp {
+                    instance.timestamp = Some(timestamp);
+                    timestamp - prev_timestamp
+                } else {
+                    // Store the timestamp from the initial tick.
+                    // (I tried to use Performance.now() to get the initial timestamp,
+                    // but this didn't seem to be accurate and caused negative dts on
+                    // Chrome.)
+                    instance.timestamp = Some(timestamp);
+                    0.0
+                };
 
                 instance.core.tick(dt);