Put a 3D play/pause on the liquid drop and keep page swipes working

The flat play/pause badge and the flash icon sat on top of the drop as two separate
symbols. With the drop on, both are gone: the icon is stamped out of the drop's front
instead, turns with it and catches the light. Pausing pops up the bars, which melt
into the play triangle; playing melts it back into the bars as they fade.

On touch only a clearly sideways drag turns the drop now, so diagonal swipes turn the
page again. A mouse released outside the window, or after turning the drop, no longer
leaves the pager dragging. The halo behind the drop drew a hard ring at the art's edge;
it is a soft radial glow now.

web/src/pager.ts

@@ -151,6 +151,12 @@ export class Pager {
   private move(e: PointerEvent): void {
     const d = this.drag;
     if (!d || e.pointerId !== d.id) return;
+    // The button came up where we did not see it (outside the window, or while
+    // the drop was being turned).
+    if (e.pointerType === "mouse" && !(e.buttons & 1)) {
+      this.up(e, true);
+      return;
+    }
     const dx = e.clientX - d.x0;
     const dy = e.clientY - d.y0;
     if (!d.active) {