Add recursive display locking under XInitThreads

[jserv]

Upstream locking.c installs a non-recursive per-display mutex when a client calls XInitThreads. It self-deadlocks the moment one locked path re-enters another: an app-level XLockDisplay followed by any internal LockDisplay, or one public Xlib entry calling another. Repoint the lock hooks at a single process-global recursive mutex so all such nesting is safe. Single-threaded clients never install the hooks, so LockDisplay stays a no-op and the lock is never touched. Release the upstream per-display lock allocation at close (previously leaked).

Serialize the SDL-queue drain under the display lock in both drainSdlEventsToPutBack and discardQueuedEventsForWindow, so XPending, XEventsQueued, and window teardown no longer race XCheckTypedEvent's locked drain on SDL_PeepEvents(GETEVENT) and the pipe wake-byte accounting.

Make the request counter (SET_X_SERVER_REQUEST) and the process-global lastEventSerial relaxed atomics; both are bumped and read across Xlib entry points and the SDL event thread.

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Summary by cubic

Make display locking recursive under XInitThreads and serialize both SDL drains and wake reconciliation to remove self-deadlocks and lost wake-ups. Adds atomic request/event counters and a ThreadSanitizer gate; single‑threaded apps are unaffected.

• Bug Fixes

  • Redirected display->lock_fns to a process‑global recursive mutex; nested XLockDisplay/internal LockDisplay and Xlib re‑entry are safe. Freed per‑display lock allocations on XCloseDisplay.
  • Serialized SDL drains under the display lock: drainSdlEventsToPutBack and the entire discardQueuedEventsForWindow now hold it, removing races with XCheckTypedEvent and wake‑byte accounting.
  • Fixed wake‑pipe lost wakeups by moving the wake byte and qlen updates under eventQueueLengthLock, and holding the same lock across resetEventWakeups’ discard‑then‑rewrite. Verified with an XTest that the connection fd wakes even with a client SDL_SetEventFilter.
  • Made the X request counter and process‑global event serial relaxed atomics (SET_X_SERVER_REQUEST, atomicLoadRequest). Added a threaded lock/queue stress test and an XTest case.

• Refactors

  • Gave file‑local globals internal linkage and documented the lock order at the mutex declarations.
  • CI: added a dedicated ThreadSanitizer job that builds with -fsanitize=thread and runs only the display‑lock threading test (LIBX11_COMPAT_THREAD_TEST_ONLY) under the dummy driver.

^{Written for commit 2d19f02. Summary will update on new commits.}