Implement automatic rotation

This change should lead to the screen automatically getting set to landscape
mode when activating fullscreen on a mobile device. Seems to only work in
chrome, which is not great, but it seems to be the only option.

multiplayer.js

@@ -1333,6 +1333,22 @@ input[type="range"]::-ms-track {
                 }
                 this.toggleButtonState(fullscreenButton);
             });
+            // This doesn't work in firefox for some reason
+            document.addEventListener('fullscreenchange', (event) => {
+                if(!matchMedia("(any-pointer:fine)").matches) {
+                    // only activate in browsers with only a coarse pointer,
+                    // which hopefully means a mobile device
+                    if(document.fullscreenElement) {
+                        screen.orientation.lock('landscape')
+                            .catch((error) => {
+                                console.error('Failed to lock orientation:',
+                                              error);
+                            });
+                    } else {
+                        screen.orientation.unlock();
+                    }
+                }
+            });
         }
 
         /*