Add WebXR demo

Based on https://docs.godotengine.org/en/stable/classes/class_webxrinterface.html
and https://www.snopekgames.com/tutorial/2023/how-make-vr-game-webxr-godot-4

Co-Authored-By: David Snopek <dsnopek@gmail.com>

.github/dist/footer.html

@@ -32,7 +32,7 @@
         <li><code>mono/*</code>: Not available yet (requires Mono-enabled HTML5 build).</li>
         <li><code>networking/*</code>: Doesn't make sense to be hosted on a static host, as the server must be hosted on the same origin due to the browser's same-origin policy.</li>
         <li><code>plugins/*</code>: Only effective within the editor.</li>
-        <li><code>xr/*</code>: Not functional on the web platform, as these demos are not designed for WebXR.</li>
+        <li><code>xr/openxr_*</code>: Not functional on the web platform, as these demos are not designed for WebXR.</li>
     </ul>
 </body>
 </html>

.github/workflows/export_web.yml

@@ -61,7 +61,10 @@ jobs:
             mono/ \
             networking/ \
             plugins/ \
-            xr/
+            xr/openxr_character_centric_movement \
+            xr/openxr_composition_layers \
+            xr/openxr_hand_tracking_demo \
+            xr/openxr_origin_centric_movement
 
           for panorama in 3d/material_testers/backgrounds/*.hdr; do
             # Decrease the resolution to get below the 100 MB PCK size limit.
@@ -89,6 +92,11 @@ jobs:
             # Enable ETC2 texture importing, which is disabled by default (but required for web exports to work on mobile platforms).
             echo "[rendering]\n\ntextures/vram_compression/import_etc2_astc=true" >> project.godot
 
+            # Enable WebXR Polyfill and WebXR Layers Polyfill for the WebXR demo.
+            if [ "$demo" == "xr/webxr/" ]; then
+              sed -i 's~^html/head_include=""$~html/head_include="<script src=\\"https://cdn.jsdelivr.net/npm/webxr-polyfill@latest/build/webxr-polyfill.min.js\\"></script>\n<script>\nvar polyfill = new WebXRPolyfill();\n</script>\n<script src=\\"https://cdn.jsdelivr.net/npm/webxr-layers-polyfill@latest/build/webxr-layers-polyfill.min.js\\"></script>\n<script>\nvar layersPolyfill = new WebXRLayersPolyfill();\n</script>"~g' export_presets.cfg
+            fi
+
             godot --verbose --headless --export-release "Web" "$BASEDIR/.github/dist/$demo/index.html"
 
             # Replace the WASM file with a symbolic link to avoid duplicating files in the pushed branch.