Drag the timeline along, and play through it smoothly

The zoomed track now drags: the handle still drags the time, while the
track around it drags the view along (or the time, with all of it shown),
a tap goes to the time under it, two fingers pinch it, and a sideways swipe
or a shift-wheel drags it too, the same with a mouse, a trackpad or a
finger. A view dragged off the handle stays put until the time moves.

Play moved in steps: the world clock told of its time four times a second,
so the handle and the globe jumped, and play stopped at every weather frame
while it drew, loaded ahead or not. The clock now tells of its time each
animation frame, and play no longer waits on the frame it is at: that
frame loads as play goes on, and play holds only at the next frame while
the one before it still loads. Weather loads its next frame ahead while the
world plays too, tells its listeners of frames as they change rather than
every frame, and the sun and moon move on each minute of world time.

src/celestialRing.js

@@ -32,6 +32,9 @@ export const KEYHOLE_LABEL_FEATHER_MAX_RATIO = 0.4;
  */
 export const KEYHOLE_OUTSIDE_OPACITY_DEFAULT = 0.01;
 
+// How far the world time moves before the sun and moon move with it: a
+// minute moves the sun a quarter of a degree.
+const EPHEMERIS_WORLD_STEP_MS = 60_000;
 const RING_INSET_PX = 11;
 const MARKER_INSET_PX = 36;
 const COLLIDING_MARKER_EXTRA_INSET_PX = 32;
@@ -452,7 +455,15 @@ export class CelestialRing {
     // A jump in the world time moves the sun and moon at once.
     this._skyDate = new Date();
     this._skyTime = new Cesium.JulianDate();
-    this._unsubscribeWorldTime = worldClock.subscribe(() => {
+    // Played through, the sky moves on each minute of world time it passes.
+    this._ephemerisWorldMs = null;
+    this._unsubscribeWorldTime = worldClock.subscribe(({ timeMs }) => {
+      if (
+        this._ephemerisWorldMs !== null &&
+        Math.abs(timeMs - this._ephemerisWorldMs) < EPHEMERIS_WORLD_STEP_MS
+      )
+        return;
+      this._ephemerisWorldMs = timeMs;
       this._ephemerisDirty = true;
     });
     this.setEnabled(this.enabled);