clock: Don't query wall_time() on the fly

We do know the update timestamp already as it is passed in via
Clock::update() and saved as Clock.last_tick.  Make use of this value
instead of calling into a browser API each time.

src/resources/clock.rs

-use crate::utils;
-
 pub struct Clock {
-    perf: web_sys::Performance,
-    start: f64,
     last_tick: f64,
     frame_delta: f32,
     pub max_delta: f32,
@@ -10,16 +6,9 @@ pub struct Clock {
 
 impl Clock {
     pub fn new() -> Clock {
-        let perf = utils::window()
-            .performance()
-            .expect("cannot access `window.performance`");
-        let start = perf.now();
-
         Clock {
-            perf,
-            start,
             frame_delta: 0.03,
-            last_tick: start,
+            last_tick: 0.0,
             max_delta: 0.1,
         }
     }
@@ -35,6 +24,6 @@ impl Clock {
     }
 
     pub fn wall_time(&self) -> f64 {
-        (self.perf.now() - self.start) / 1000.0
+        self.last_tick
     }
 }