Keep a playing video when the window comes back to the front (#402)

Chrome draws no frames for a hidden page, so the scene stopped asking
for its videos. The sweeper of the VideoLibrary kept running and
dropped every player after 2 s. Back in the window, the video started
over, and Pixi warned that a texture source was destroyed while still
bound to a shader.

- The sweeper skips a hidden page. The time hidden does not count as
  idle, the 2 s start once the page is shown again.
- A dropped player only unloads its video source instead of destroying
  it. A shader that still holds the source no longer warns.
- CI serves videos through a new router for php -S. The built-in
  server answers no Range requests, so Chrome could not seek. e2e 66
  had 4 red checks in the suite job of d48f174 because of it. nginx on
  stein2 hid this.

Tests:
- Unit video-library.test.ts (new): 3 tests
- e2e 66 (atx): ALL PASSED, 18 checks
- Router checked with curl: 206 with the right bytes
- Counter-check: the old sweep fails both hidden-page tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/ci/router.php

@@ -0,0 +1,45 @@
+<?php
+// Router of the CI jobs' dev server (see serve.sh). `php -S` answers no Range requests, and Chrome
+// cannot seek in a video served without them (#402): a clip stayed at the video's first frame. On
+// stein2 nginx does this. Here the converted videos get their ranges; everything else is left to
+// the server as before.
+
+$path = rawurldecode((string) parse_url($_SERVER['REQUEST_URI'] ?? '', PHP_URL_PATH));
+if (!preg_match('#^/game-maker/data/videos/[0-9a-z]+/[0-9a-f]{64}\.mp4$#', $path)) {
+    return false;
+}
+$file = $_SERVER['DOCUMENT_ROOT'] . $path;
+if (!is_file($file)) {
+    return false;
+}
+
+$size = filesize($file);
+$start = 0;
+$end = $size - 1;
+header('Content-Type: video/mp4');
+header('Accept-Ranges: bytes');
+if (preg_match('/^bytes=(\d*)-(\d*)$/', $_SERVER['HTTP_RANGE'] ?? '', $range)) {
+    if ($range[1] === '') {
+        $start = max(0, $size - (int) $range[2]);
+    } else {
+        $start = (int) $range[1];
+        if ($range[2] !== '') {
+            $end = min($end, (int) $range[2]);
+        }
+    }
+    if ($start > $end) {
+        http_response_code(416);
+        header("Content-Range: bytes */$size");
+        return true;
+    }
+    http_response_code(206);
+    header("Content-Range: bytes $start-$end/$size");
+}
+header('Content-Length: ' . ($end - $start + 1));
+if ($_SERVER['REQUEST_METHOD'] === 'HEAD') {
+    return true;
+}
+$in = fopen($file, 'rb');
+$out = fopen('php://output', 'wb');
+stream_copy_to_stream($in, $out, $end - $start + 1, $start);
+return true;

tools/ci/serve.sh

@@ -40,8 +40,8 @@ game_maker_serve() {
   # game-maker-video, here a process beside the server.
   php "$root/tools/video-worker.php" "$root/data" >/tmp/video-worker.log 2>&1 &
   # Without workers the single-threaded dev server stalls: the editor fetches project and
-  # textures in parallel.
-  PHP_CLI_SERVER_WORKERS=8 php -S 0.0.0.0:8080 -t /srv >/tmp/php-server.log 2>&1 &
+  # textures in parallel. The router serves videos with Range requests, see there.
+  PHP_CLI_SERVER_WORKERS=8 php -S 0.0.0.0:8080 -t /srv "$root/tools/ci/router.php" >/tmp/php-server.log 2>&1 &
 
   for _ in $(seq 60); do
     curl -sf "${GAME_MAKER_BASE_URL}api/index.php?r=auth/session" >/dev/null 2>&1 && break