Replace per-pipeline QThreadPool with a shared static pool to prevent
file descriptor and thread exhaustion. Each GstEnginePipeline was creating
its own thread pool, leading to resource accumulation during frequent
pipeline creation/destruction (track changes, seeking, crossfade).
The shared pool is limited to 2 threads max since state changes are
typically sequential per pipeline. This prevents the crash in g_wakeup_new()
when creating eventfd for new thread event dispatchers.
Fixes#1687
c684a95f89