Add 3d Viewport component

Implement a Viewport component and associated update script that allows
rendering 3d content inside it.  This is based on the implementation
used at Furnavia for their 2025 website.

web/style.css

@@ -64,6 +64,19 @@ ol, ul {
 	padding-inline-start: 1.25em;
 }
 
+/* 3d viewport */
+.in3d {
+	transform-style: preserve-3d;
+}
+.viewport {
+	pointer-events: none;
+	perspective: 300vmax;
+	perspective-origin: 50% var(--y-offset);
+}
+.viewport * {
+	pointer-events: initial;
+}
+
 /* Base Page Layout */
 body {
 	max-width: 50rem;